We've written to ServInt and are awaiting their reply. However, given the many posts about ServInt we've read on WHT, we'd like to additionally obtain some feedback on this specific security issue from existing customers, if possible.
I've only been with ServInt for a few days, and this is my first time with anything greater than a reseller account...so take the following knowing that I'm a newb
I would answer your questions as:
A software firewall like APF is not preinstalled on VPS accounts, primarily because, as I understand it, it is not completely compatible with the VPS kernel (ServInt uses Virtuozzo to power their VPSs). However, APF will work, just certain parts of it won't. I had, in fact, tried to install APF soon after I got my VPS account only to run in to some errors. I then visited ServInt's forums, found detailed discussions about using firewalls with a VPS, followed the NOC's instructions for installing APF, and had it up and running in minutes.
Reed, the founder of ServInt, will be the first to tell you that a firewall isn't required for a secure server...but he'll also be the first to help you in setting one up.
I'm sure you'll get a more detailed and accurate response from ServInt, but there's my $.02.