WordPress can sometimes use 32M to build a page.
That kind of setup is only useful where most of your visitors are NOT logged in, since logged in users often cause cache-misses.
It actually ends with MySQL + Nginx + PHP + WordPress all running in addition to the 18 static sites, which does seem a bit unlikely to hold up very well, even if the setup shown does leave the VPS with 48MB free.
WordPress can sometimes use 32M to build a page.
That kind of setup is only useful where most of your visitors are NOT logged in, since logged in users often cause cache-misses.