How many tons of force do you need to hold the vise down? And,how many tons of force does it take to jerk a hole in your T slot?
Of course,if the T nuts are directly beneath the slots in the vise,as they should be,the studs will only be compressing the cast iron of the table between the T nuts and the vise AS IT SHOULD.
I think 3/8" would be adequate. If your clamping set has only 3/8" bolts,and the T nuts fit your table,the studs are the maximum size that will fit through your table's slots.
Cheap T bolts and nuts aren't really as strong as they should be anyway. Their threads get distorted and pulled sideways. A proper,quality clamp set like a Teco has mjuch better,properly hardened T nuts and studs in it that can take a lot more torque.
They had a Teco set at Fastenal that was $250.00. I was able to buy the identical set on sale from Enco for about $55.00 several years ago. Unfortunately,the 1/2" stud size sets are much cheaper than the 3/8" sets because they are made for Bridgeport size machines,and are much more common. I had to pay much more for a 3/8" set for my Harrison universal mill because they make fewer sets of those. I haven't had trouble with the vise walking on the Harrison,and it is a very powerful,geared head mill that can move more metal than the Bridgeport.
I do advise either making or buying some heavy,1/8" thick hardened steel washers to place under the nuts you are tightening down the vise with. Ordinary washers will just cave in and will not be capable of using the full force of your clamping studs.