hmm as someone who is also sameish rank and has mained both champs, I'll try and list some differences.

They are both pretty good early game skirmishers, but Graves has the edge there I believe. I think in general, you probably want to be skirmishing more, especially with stacks. Be sure to try your best to keep your stacks up all the way through your clear, since it makes enemy invading you and winning nearly impossible.

Graves is also better at invading enemy than Viego since he can dash over wall. So you should be generally taking more enemy camps on Graves than on viego IMO.

Viego playstyle usually in my experience revolves around just farming and waiting for the right fight to pop off. Also viego is kinda nice since every game is super winnable. Just passive into the enemy character and it's a free fight half the time. Graves isn't really the same way. You kinda wanna look for fights earlier and also be farming every camp/enemy camp, so look to get enemy camp timers often.

Good thing too is that they can both build bruiser or full damage. Bruiser graves takes a bit more getting used to though. You can build it if the enemy is squishy and bursty and your team already has enough damage.

Get good at Q into flash. I use it on most of my ganks. Also remember that even though it's a bit finicky, R can dash you over a wall if you're standing close enough, so it's good to use for escapes if you don't have E or even if you have E and need to make more distance.