I suppose it’s possible. The fact that she can super-jump gives her the ability to run away from certain situations. Doing tiger-kneed Burning Kicks so you are flying away from the opponent creates a lot of space for you and sometimes the opponent will get impatient and try to follow. That’s when her Seismo game comes in handy.
She can also escape corner traps by super-jumping forward and doing EX Burning Kick at the peak of the jump at the right opportunity.
As I said before, sometimes her defense is set up by her offense. You can use her feints to freeze the opponent and run away afterwards, or you can entice them to jump and punish afterwards. Use her ability to High-Jump Cancel to run away and jump backwards to reset, either after hitting with normals or with Seismos.
Sometimes I am forced to play really defensively against certain characters like Zangief, Honda, Blanka, Claw, and Boxer, so it’s actually a necessity to learn how to play defensively. However, it’s good to have that balance and know when is a good time to play aggressive. Mixing it up gives Viper the best chance to win IMO.
The only characters that you can’t play defensively against is maybe Ryu and Sagat, because their fireball traps will eat you alive if you try running away. However, it’s a tough call because for both characters, all they need is to pop you up with srk to deal huge damage to you, especially when they have Ultra stocked. If anyone had success fighting them very defensively, I wish I knew how they did it… but I think it’s actually near impossible.
Anyways, I think it’s possible to be a defensive Viper. Just as long as you can punish people when they make mistakes, you’ll be fine.