I use a VPS for n8n as well however all of the automations there are form-submission triggered, so I simply connect the Webflow form to a webhook URL as the action.
I do remember needing to do some form of OAuth from n8n for a different service ( Google drive maybe ), and it was messy, I found some n8n docs or forum posts that helped me figure out the process, there was a lot of manual hacking involved to get the OAuth chain to respond.