I am going to use Serverpilot as a panel, which is a remote panel which takes nearly no resources and allows PHP versions down to 5.4 which works OK with my version of phpbb.
page views per month is irrelevant
How many simultaneous visitors do you have?
When you say "simultaneous visitors" do you mean peak concurrent visitors on the server, or what phpbb is reporting as "users online"?
50.000 page views means on average about 1.6 page views per minute, which at peak hours would at most be 6 page views per minute, so 1 visit every 10 seconds. So there would be rarely any concurrent visits and when it happens it wouldn't be more than 3 or 4.
"Users online" as reported by phpbb is irrelevant. For my forum a "user online" is based on users active over the past 5 minutes, for this forum for the past 90 minutes. A user can be making 1 page view or 100 page views in those 5 minutes. What matters to server load is the number of page views. Certain actions, like making a post, could require more resources than simply viewing a page, but my forum receives very few new posts each day, so that wouldn't be an issue either.
The people to ask would be the hosts, we only supply the software.
I am basically asking other users who have already tried this. E.g. if somebody is already using such a VPS to run a forum which is more active than mine, that would mean it would be fine for me also. Or maybe somebody tried it with a less active forum but it still wouldn't work etc.
So basically I need an answer from people who are running phpbb on a VPS