FWIW, I think that this is an impossible question to answer via the forum because there is no way to analyze what you are actually doing.... for example in your first post we were talking about "a site" as in ONE site... now we find out that your are talking about 9 sites on one VPS!
also for FWIW...all wp "sites" are NOT created alike, especially with wordpress.... all the various plugins can cause the site to behave differently..
it greatly depends... if 15,000 hits come evenly distributed over X.. with X being a day or a week then there is one answer, but if all 15,000 hits come at 7PM in a 5 minutes window then the answer will be entirely different answer....an answer that will waste alot of rsources for 23 hours and 55 minutes just to cover the one 5 minute period that gets all the traffic.
That example is not real-world but given to illustrate the point that how the traffic is distributed also affects the answer.
If i were you, nne thing I might try to get to the bottom of this is to edit robots.txt to find out if the search crawlers are indexing too much too often.
There is a double edged sword there however, if your posts are "trending topics".. one the one hand you want the crawler to find the new content quickly (which means you don't want to issue instructions via robots.txt to only come back once a week) , on the other hand if your site is huge, you don't want the crawler coming back and re-indexing the sites in total every 24 hours.
I see your choices as being 2-fold-- either analyze your traffic and understand it, and try to fine tune the bot traffic
follow the recommendation that support already gave you and raise your 600mb VPS to 900-1000MB......