What software is preinstalled? (Apache, MySQL, … ?)
Sendmail is pre-installed.
How do you access your VPS?
Via SSH, you can utilize PuTTY, or the SSH tool of your choice.
How many domains and subdomains can be hosted on a VPS?
It depends on how many apache configs you want to put in, as there is only a basic stop/start control panel, not one for doing domains and subdomains for VPS config.
Do you manage the security on the server?
We do not provide the basic lockdowns you may need on the VPS, it is your job to do this as we allow you SSH access. If you have a particular problem we can offer some assistance via the ticket system to help you.
Do you do some kind of monitoring of the web and db server running on a VPS or is this the clients job?
We do not monitor the VPSes on an individual basis, but do monitor the host server to ensure it is up and the VPSes it contains have started.