I have a main enemy in my story I want to use. As a way for him to be immortal, I was going to make him a vampire. I want him to interact with the party but they have a paladin who can detect evil pretty well. I was wondering if a vampire can conceal his evil aura?
He could have a Ring of Mind Shielding. 8,000g for a ring that makes you immune to any attempt to magically discern alignment, plus it gives immunity to Detect Thoughts and Discern Lies.
Sure, there are a few possible ways. The real question is do you want to hide the fact that you are evil or not be evil at all?
1. Undetectable Alignment
Using a wand of undetectable alignment would cost 4,500gp and give 50 days worth of hidden alignment. If you can manage the ongoing cost then this is a way of handling the problem even at low level.
Greater Magic Aura could work too. It's a higher level (and therefore more expensive) but let's you detect as you like and also masks a number of different things that may be important to you.
2. Magic Items
The Ring of Mind Shielding does the same job too, with the added bonus of being immune to Discern Lies and Detect Thoughts. All for an affordable 8,000gp.
The Plaguebringer's Mask gives a number of minor benefits and also makes you appear to have a Neutral alignment for a meagre 2,000gp.
If you get to make your own magic items then you can make an item which has continuous/user activated Undetectable Alignment for 12,000gp or if you only want one charge a day, which should be sufficient, then it can be as little as 2,400gp.
Mask of Virtue is a feat from Champions of Corruption which masks your Alignment.
4. Special Materials
Angelskin Armour will mask an Evil aura of less than 10HD. So as long as you aren't too high a level then it is great. It's a mere 1-2,000gp as well!
5. Prestige Class
Taking two levels in Master Spy will; let you detect as any alignment you like.
Greater Change Alignment can make you Good permanently, but you need a 20th level Alchemist to help you out!
7. Don't be evil
Alignments are defined by the characters, not the other way around. If you want to stop detecting as evil then stop doing evil things. Adopt a puppy, do charity work with the poor and fight bad guys - but you really have to mean it! The game covers what is necessary for a change of alignment, and characters moving to Good will need confess and atone for their past ways.
There are also some shorter-term solutions such as Misdirection that require regular upkeep or may not last long enough, so have been generally omitted.
Ther most obvious way to hide one's evil aura is Angelskin special material from PF.
The preserved skin of an angel retains a portion of celestial grace and can be crafted into leather, hide, or studded leather armor. Angelskin radiates a moderate good aura that masks malign auras. Any evil aura radiated by the wearer is reduced in strength by 10 Hit Dice. Auras reduced below 1 Hit Die can't be detected by means such as detect evil; the creature doesn't detect as evil, though this has no effect on other aspects of the creature's alignment. For example, a weak chaotic creature wearing angelskin armor detects as chaotic, but not evil.
All of the above answers are more functional, but I wanted to add another possible option: the tiefling's do have a variant racial ability that allows you to choose the outcome of anything attempting to detect your alignment (number 70 on the Variant Tiefling abilities chart).