And you should never assume your site is safe. The best secured sites in the world can be hacked given enough effort. It's very difficult to evaluate the relative security of a site that is one among hundreds hosted on the same machine.
If your host recognizes the difference between a server-wide IUSR account and user accounts unique to each virtual domain, then you're probably safer than most of us. But, did the host leave the default "Full Control" rights for the "Everyone" group in the C:\Windows\System32 folder? If so, any user able to view your site can do anything they want to that critical system folder. The list of exploitable mistakes is endless.
The most that can said about a host that uses virtual domain hosting best security practices is that WHEN one of the sites on the machine gets hacked, only that site can be hacked and the hacker can not then use that site to hack the machine or other sites on the machine.