... maybe? I spent a LOT of time looking at grappling under the 5e24 rules. It's a lot different than 5e14 even thought it looks similar at first glance.
It's not exactly an "attack" and advantage, AFAIK, doesn't help. Instead, think of the following flow chart:
1. You declare an unarmed strike action.
2. When you declare an unarmed strike action, you have the option of a) damage, b) shove, or c) grapple.
3. If you choose damage, you make an attack roll against the target.
4. If you choose grapple (or shove)... there is no attack roll. Instead, the target must make a save (either S or D, target choose). The DC for the save is 8+str+prof.
In other words, having advantage does nothing for grapple. On the other hand, imposing disadvantage on the target (for saves) is helpful.
For that reason, vex would do nothing for grapple. Prone also doesn't affect the saves.
If you are going with a strong grapple build, I really recommend reviewing the changes to the rules thoroughly. They appear the same as 5e14, but they really, really aren't. Now, I might be misreading all of this, but you might want to check the text again - I mean, grapple uses an attack action, but it's not an attack roll based on my understanding. If I'm reading it right, and you want to grapple more successfully, look for things that target the opponent's saves, and remember they get to choose between two (so even if you impose disadvantage on, inter alia, strength, they can always choose dexterity).
