I'm not sure what profiles you're talking about exactly, so a definitive answer cannot be given. It depends on the site, some sites requires you to log in again, if you switch ips, others don't.
I personally wouldn't use VPN for this, i'd use proxies. iMacros natively supports proxies or you can just configure your browser to use a proxy either via settings or with an addon like FoxyProxy. You can set up multiple Firefox profiles, portable ones, so you can use them separately and the proxy settings won't affect each other, so you can run 5 instances in the same time and they won't interfere with each other.