It shouldn't make that much difference if the Federation spans thousands of lightyears with dozens of members species. Whats a few billion Vulcans if the total population of the Federation is hundreds of billions with thousands of colonies?
Actually, the Federation of the TOS/NuTrek era shouldn't be all that big, consisting of only a couple of HUNDRED worlds and their colonies, with most of the powers being minor. Losing Vulcan would be a huge loss to the Federation outright.