drupal site is really slow on VPS server
Isn't it closer to 4Gb RAM based on your posted stats?
A server with only 1Gb RAM will most likely not work properly with a full application stack. Apache and MySQL require memory to be performant. The least I'd recommend based on personal experience is 2Gb RAM.

What you can also do is perform traces on your site with xhprof to see what the slowest items are, and improve those.
APC and memcache can also help to boost speed. Or if you only have anonymous users, using the boost module can be a very quick fix, although it's not maintained actively anymore for Drupal 6 it seems.