If you got traffic, not much, average, a default my.cnf is enough to consume your total ram by itself... Upgrade to a bigger plan or minimize
Apache startservers, set MaxClients to a lower number and MySQL max_connections to the same number as MaxClients. Below 100 for sure...
Stop clam if it's running. Spamd too if spam doesn't annoying you.
Minimize php.ini memory_limit too. Timeouts also. On both php and apache.
Don't forget that Centos barely can update it self with 128MB ram and you are running apache + mysql + email server + dns + panel + + +.. on a 376mb ram vps.
If upgrading is not an option, did it use swap ? If not, it may just cache the ram. If yes and heavily, create a second bigger swapfile. It's not really a serious option to do, but you can do it...
Btw, is cups/cupsd needed to run on the server? ALso,
a. if we disable webmin from the bootup, will we be still be able to access it via the virtualmin panel?
b. if we disable yum update, will running the yum update command in linux give us the same result ie. searching for new updates?