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..
Just visiting more or less weekly as this is a side interest to me (CP man myself) so just don't much to contribute unless I make a trip and run into the plants. No problems with ads, but in general most fora are going under anyway due to FB etc.. Few die hards refuse to let go but unless another member brings them the stuff from FB and twitter (Boy do I hate that blue bird) they start missing out on news. People want instant answers these days and that you can't search is not an issue.they just ask again and there is always someone who will answer...