A pageview count alone isn't too useful for this question, for the various contextual reasons John already brought up. But if we just go with it anyway consider that 50,000 views a month is only ~1700 per day. If a shared account can't handle that, yes it's a hint the current hosting might suck(that's the technical term). But it says almost nothing about needing to move to a VPS. It just means you should get a better–but still-shared–account/host.
While VPS accounts do generally come with higher resource limits, for CPU if not necessarily storage, I personally think the more important factor in choosing one is the control you get. eg. root access and so on. If you don't need those things, a good shared host can take care of you for a pretty long time. And if you don't know how to administer an entire server environment, don't even consider it as VPS plans usually come with very limited technical support. The entire point of them is "here's a server; do whatever you want with it." It's possible to find host-managed VPS setups, but they'll be significantly more expensive.
Really, there are so many potential variables here the only good answer is "move when your current setup can't handle whatever you're throwing at it." With MediaTemple's grid, for example, there's a fixed amount of their service you're allowed to use per month(1000 GPUs), and no upgrade path. If you want to stick with MediaTemple, you'll need to move to a dv plan or higher. I had a client in this situation that I moved to another quality shared host instead, and their system's had no trouble at all handling the site, even as traffic, bandwidth etc. continue to grow. A trawl through performance will turn up various threads on how to actually go about evaluating your system's performance.
