@DrWizardo
So Vantage:
She is actually pretty good, although clunky as you say.
@LaughingSharko is correct with the use of Echo.
I tend to use Echo to push fights, especially after my teammates have already pushed and I have hit one or two of the opposing players with Snipers mark.
With that in mind I also tend to play Vantage quite aggressive, using CQC weapons, like having an R-99 or Alternator paired with another SMG or AR weapon. for me this is great because I can use Snipers mark to get in some initial damage as my teammates get closer and then use Echo to get up close and personal with the CQC weapons.
Also Echo can be used to bait other teams and that is a great use. Plus as @LaughingSharko said she is already out for your reposition and or a possible exit Strat if needed. The one thing that is great about Echo is that the double jump can be used to change direction or even to perform a 180, something Loba cannot do.
The main thing as with all other Legends is that your teammates have to work with you. I play a bit with two friends so we have our strats pretty well set, I also play a lot solo and while that can be more difficult, with using her sniper abilities and when to push/not push it tends to work out.
Snipers mark is also pretty good at keeping other teams at bay with some pretty decent poke dmg especially if you can land that second shot or if one of your other teammates has a sniper weapon as well and can provide the time needed to rotate or reset. It is also great for picking off straggling and or struggling enemies that have been 3rd or 4th partied.
I am also just another casual player as well, even though I have basically mained Vantage for the last season and a half, and have found that with the CQC set up paired with Snipers Mark works well with my play style.
GG's all.