Difference in soy milk?

Alexia, for the future, there is a brand of unsweetened vanilla soy milk that is excellent for baking.
WestSoy, Unsweetened Vanilla. You can find it at Trader Joe's, WholeFoods, even my Kroger and Meijer both carry it. Look for it with the cereal, not in the fridge.

Silk brand original is the same as any other plain soy milks

In my experience "plain soymilk" refers to unflavored sweetened soy milk of any brand, Silk original counts as such.
Even if it were vanilla flavored I wouldn't worry too much as you are making cupcakes, it would make a lot more of a difference in something like quiche, where sweet vanilla might be very odd.

Unless you buy 'unsweetened' soymilk, other soymilks have added sweeteners, to mimic the lactose sugars in dairy milk, so the sugar content in 'regular' forms may differ from each other, but only in degree. Vanilla is distinctly sweeter tasting.

Yes, there will be a difference. Silk is sweetened-- so you will need to reduce the amount of sugar called for in the recipe. You can leave it--they may be overly sweet, but probably still okay.

If the recipe calls for 3/4 c sugar for a dozen cupcakes, i'd reduce to 1/2 c to compensate for the sugar in the soy milk.

Silk is just the brand. As long as it isn't flavored with vanilla you are fine.