Can the Grappling Strike maneuver option be used with the Thorn Whip cantrip to grapple a creature from up to 30 feet away?

Question

Grappling Strike is a maneuver option for Battle Master fighters (and others with access to maneuvers) that reads (Tasha's Cauldron of Everything, p. 42; emphasis mine):

Immediately after you hit a creature with a melee attack on your turn, you can expend one superiority die and then try to grapple the target as a bonus action (see the Player's Handbook for rules on grappling). Add the superiority die to your Strength (Athletics) check.

Unlike other maneuvers, it doesn't specify that you have to be making a weapon attack (probably so it will combo with the Unarmed Fighting fighting style that was printed in the same book).

Thorn whip is a cantrip with a range of 30 feet and the following description (Player's Handbook, p. 282; emphasis mine):

You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range. Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.

Despite having a 30-foot range, thorn whip counts as a "melee attack".

Can a character with access to both the maneuver and the cantrip use their action to cast thorn whip, and then spend the superiority die and a bonus action to attempt a grapple from long range, assuming the other requirements for attempting a grapple (such as needing to have an empty hand) are met?

Answer 1

No 30 ft. grapple

As you identified, Grappling Strike requires a "melee attack" not a "melee weapon attack", and thus Thorn Whip can satisfy that requirement. However, it goes on to say:

try to grapple the target as a bonus action (see the Player’s Handbook for rules on grappling)

In other words, other than the specified differences (uses a bonus action, adds a bonus to the check) it follows the normal rules for grappling which include:

The target of your grapple must be no more than one size larger than you and must be within your reach.

Neither Grappling Strike nor Thorn Whip removes these requirements or extends your reach, so if you cast Thorn Whip on a target 30 ft. away, you will be unable to use Grappling Strike afterwards unless you have a lot of reach from some other source.

Yes* 15 ft. grapple

In addition to the melee spell attack, Thorn Whip also can pull a Large or smaller target up to 10 ft. closer to you. A typical Medium adventurer can grapple Large or smaller creatures with a reach of 5 ft., so using Thorn Whip on such a creature can bring it into grapple range while also satisfying the "hit with a melee attack" condition of Grappling Strike. The specifics can change a bit based on your circumstances; for example, a Small race would not be able to grapple Large creatures this way, while a race with longer reach (I think Bugbear has this?) would be extend the combo to 20 ft.

*Note that the result is a grappled creature adjacent to the player, not at their original location. If you were looking to keep an enemy in place but far away you will need to look elsewhere.

Answer 2

This does not work

The grapple rules state:

[...] The target of your grapple must be [...] within your reach. [...]

In this case, the target of your grapple is 30 feet away from you and thus is not within your reach. And even if you could grapple the target because the feature somehow makes an exception of some kind (I don't believe it does), the grapple would immediately end as the target is outside your reach. This is because the Grappled condition states that:

[...] The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect [...]