A single VPS set-up might be OK for serving content over web, but in my experience, the pain begins when your software starts doing async processing - long-running cron jobs, queue processing. If you're doing it on your web server machine, there will be downtime.

I know this, because I have gone through these issues with each of my projects. Just recently an infinite loop bug in a cron job ground my "single VPS" setup to a halt (and took the web server with it).