There is software that can do that.
Look here: https://cms2cms.com/cms/supported-cms/vbulletin-to-phpbb-migration/
It seems to be an automated proces so little effort.
It has a calculator that gives you costs based on pages, topics, comments and number of users.
As this forum is not really big it might be affordable...
20 pages and 100 users comes to an estimate of 19 dollar as an example.
It might be worth deleting inactive registrations and non important comments though..