To clarify for those who are confused, upgrading the number of virtual CPU cores on a VDS will not make it any more able to run a single 64-slot server. If you have a 2-slot VDS and upgrade it to an 8-slot VDS, it still might not be able to handle even one 64-slot server. This is because the individual core performance is mostly what matters, and you can't increase that performance by adding more cores.
With more cores, you could run more servers, though, of course. Also, if you have spare cores, try the "Multithreading" option on the "Managed panel" page, which may improve allow a few more slots to be run without lag.