Yup probably would - and to be honest, you won't see the difference on a hand. SI IIRC is visible to the naked eye but you have to work hard..... VS is one of those Jewellers eye telescope things.....
SI from what I read you still need x10 magnification (or at least one helluva eye) to see imperfections, VS would be difficult to tell even with an eye piece).
but £800 difference?
if so then sod it SI will do, it's more about the stone in the centre I think.
AAA tanzanite will look nice, colour is lovely.
Most people would easily see inclusions at I Grade
Most people would see Inclusions at SI with a jewllers 10X loupe and trained eyes without, I could pick up on an SI
VSI a trained grader would need a Loupe
Anything above that is prestigious
The clarity does in fact affect more than you would think. The brilliance and fire generally increase the higher the clarity grade.
Would I pay more for VSI as an accent? No, they are only accenting stones not the main centrepiece so i wouldnt spend the extra £800 personally.
If you are going to get a diamond with an EGL certificate, check what the Cut grade is. Generally, most people think this is just about the shape but theres a whole new world when you go into depth about the cut. A great clarity stone with a poor cut will have the approximate similar appearance of perhaps an SI with an excellent cut.
There are many factors that increase a diamonds value, but all in all if its only as the accent stone i wouldnt shell out the extra £800.