The answer to the main question is a solid yes, but we need to be careful of how we're defining effective
What situations will this character be encountering?
The first and most important element in thinking about this is about your campaign. If it's a very plot-heavy, on-rails scenario which brings PCs into contact with vampires under vampire-friendly conditions, a rootstock human isn't going to seem very good.
Even a weak vampire gets huge advantages over kine, most immediately including spending blood to pump physical Attributes, resistance to bludgeoning and lethal damage, and various supernatural powers with a huge variety of effects and often few counter-strategies. Dropping some generic guy into a room with some generic Kindred for a direct, 1-on-1 competition of any kind is a situation in which it would be unwise to bet on the human.
That's not how Kindred operate in V:tM. No vampire with middling physical Attributes is going to blithely walk into a confrontation with a vampire that has high Potence, Fortitude, and Celerity ratings. Kindred always try to play to their strengths and mitigate their weaknesses, and so avoid situations where there is an imbalance of capability which is not in their favor.
Good strategies for a human character could involve things like operating during the day, or exploiting a vampire's instinctive fear of fire, their (sometimes) better understanding of current social and political environments, or their better connection with Humanity (owing to a weaker Beast) to expose vampires to weaknesses that the human doesn't share. Allowing themselves to come into a 1-on-1 confrontation with a vampire without extensive preparation to "level the playing field" is behavior of a human character that isn't going to survive.
In short, a human character could possibly be effective in a campaign with vampires as allies and enemies. But that character will almost certainly not be successful if played very much like a character that has been Embraced.
The Revised-edition sourcebook Ghouls: Fatal Addiction has some great sections on why vampires might rely on non-Embraced servants and assistants (which can give some inspiration for things your human character might be focused on, and how they benefit the group). It also has great sections peppered throughout about how humans (ghouled, particularly, but with some general insights applicable to mundanes as well) interact with the schemes of the Kindred. V20 has carried much of that material forward with Ghouls and Revenants, but I recommend looking through Fatal Addiction as well; I thought it was one of the better-written and more useful sourcebooks released across all of Revised.
Combat in the Storyteller system favors luck, and a human can be just as lucky as a vampire
The second important element to consider is how skill checks, particularly in combat, work. Combat in V:tM is very brittle and swing-y, and while that holds at least as true for a mortal character as an immortal one it's worth bearing in mind that vampires have farther to fall when it comes to injury-based dice penalties. It doesn't matter as much if an enemy has blood-buffed their Strength to 7 dots when enough hit boxes are marked off that they have a -3 or -4 penalty. Even a small penalty can convert likely successes to marginal failures, and for skills that's a big deal.
A couple of bad rolls when defending, or an overwhelming attack, imposes some pretty severe dice pool penalties, and those penalties can set in pretty quickly. One of my players in a V:tM game had a modest number of dots in Firearms and had a shotgun as his main weapon. A shotgun blast can deal a huge amount of damage in a single attack, and doesn't rely on superhuman abilities to use that way. Something like a flamethrower, or Molotov cocktails, could be even more effective.
Being able to deal heavy damage won't necessarily help your human survive, perhaps not even helping enough to let them get off a single attack, but if they are able to attack they can contribute Kindred-level damage in many cases, and the injury penalties to dice pools can remove a lot of the advantages a vampire would have in combat (from a game-mechanical perspective).
Special abilities favoring humans and/or frustrating Kindred exist
A third consideration is that there are some, though not a large number, of options a human character might take to give them unusual resistance or immunity to Kindred powers. The most direct such trait I can think of offhand is the True Faith merit.
It's expensive, but gives options to deal aggravated damage and resist some Discipline powers like Presence or Dominate. It was explained in one of the core sourcebooks for the Revised edition that regular people with True Faith were a serious, existential threat to vampires of any level of power or influence in the early days of the Common Era.
Some other traits exist, with variable efficacy. To give a human character a slight extra boost, a Storyteller might allow them to take other Merits like Unbondable or False Aura, among others, which can frustrate vampires' efforts to interfere with the human supernaturally.
The Revised-edition sourcebook Hunter X gives some extra options for making a human character that can offer some meaningful opposition to the Undead, but also underscores that even with the best advantages available a human who regularly contends against vampires is leading an extremely dangerous life.
Groups succeed where individuals fail
The final consideration I recommend is one any coterie should consider: what is the human's role in the group? If the human and vampires are working together, there must be some contribution that the human makes (or can make) to their efforts that would justify flirting with the edges of the Masquerade. And even the most powerful and influential vampires tend to build organizations (including informal collections of dupes) to pursue their goals. Your human can contribute to the success of your group, and can also help in dismantling an enemy's organizations.
The exact niche the human fills in your coterie is something I can't describe very well, not knowing any of the details, but as above vampires that survive for long (and humans interacting with vampires that survive for long) will tend to understand where they're strongest and where they're most vulnerable, and seek to account for those. If an enemy vampire's major advantage is wealth, or Contacts in the police department, those are threats that a human can face which will not offer the same kind of dangerous resistance as the vampire controlling those things would.
A coterie with a human member is extremely unlikely to have that human charge forward as the vanguard in a battle against a vampire. That's simply something that even the least combat-oriented vampire in the group is likely to be able to do better than the human. Figuring out what special advantages the human member does offer will define what things that character does and does not do.
I tend to feel that human characters (ghouled or not) offer unique advantages in maneuvering around outside of combat. At least, that's been my experience with human NPCs used as Retainers, Contacts, or Allies in V:tM games. A vampire will go to some trouble to protect such a character in a fight not because the fight is where the human matters most, but because the human is in some way a resource worth the effort to protect. That's not being a drag on the coterie, it's a fun in-game constraint which allows the other PCs access to the human's unique and valuable capabilities (whatever those specifically may be in your campaign).