Tanks in Online mode can almost only be countered with other tanks, so it would be smart to get your own one... in case someone wants to grief you, you can grief him instead.
If you want to parachute, you could go to the airport and get yourself a Luxor instead of a Vestra, a tank is quite unique ... so if you're short on money, I can only give you the advice of buying your own tank.
However, you could save some more money and buy the Buzzard... GREAT for missions. But since you only asked for our opinion on the tank vs. Vestra, I vote for the tank.
The Buzzard would be better against a Rhino Tank because of it's ability to hover over the tank and shoot rpgs while being out of the tank's range.
I don't know but I never got even hit by a Buzzard while tanking. Even a jet never hit me, but I took down hundreds of choppers and about 30 jets.
They are just flies that are annoying, but not lethal at all for the tank