I have both the 20th Shiva and an Ecstasy 101B. For Vai's tone, sorry to recommend this, but you'll get a lot closer with the XTC. I remember when I first got mine, I thought, "wow... I always thought those Vai-type sounds were coming from his fingers." Technically, that's still correct, but a LOT of it is the XTC. It sounds like nothing else and remains my favorite amp for leads.
Overall, the 20th Shiva is darker, thicker, and heavier sounding. It's an amazing amp to be sure and covers a LOT of ground, very versatile. Great clean, great crunch, even gets almost as brutal as my Uber. It's great for a lot of 80s sounds, and is a great amp to play lead with. However, other than being a Bogner, the XTC is a really different amp. Apples to oranges.