ah... ok. I don't think there's an easy way to do that. You could probably get tricky and tell php to read a different config file than php.ini, but that could cause a host of other issues, and I doubt you'd have much luck setting that up with out sudo privs.
As for a vps, I use Linode now and I've used Slicehost (I think Rackspace bought em, though) and Servint in the past. I mention them all, because they all worked and they all had plans where you could start with a very small vps account and upgrade it as necessary. Upgrading it leaves all your stuff in place; you just have to be down for a little while if you upgrade to a larger plan, as they move all your stuff to a different server. Usually takes less than an hour after you telling them that you need a larger plan, so I usually err on the side of a small plan and then raise the specs as necessary. What you need is really determined by factors I don't know... how many concurrent users, what are they doing in terms of moving bits around, etc.