If I were at your position and the product is actually ready already I would spend all of that cash on advertising after I would make sure that my current server infrastructure can handle the possible growth.
Though first I would calculate how many new visitors I possibly could get from adwords with the cash. Once the calculation is done I need to test if the possible growth can be handled. If I can not handle the growth I would check how much new hardware I would need and buy and install those. The rest of the money would be used in adwords for new possible clients to create more revenue.
I would not have used the money to get new computer or hired virtual assistent, because I see that as a waste of money. Then again I would not even bought that VPS as I would have one or two dedicated servers anyway.