Hi - AppSource can be a good starting point, sometimes the per user license cost of subscribing to these apps may be too high. Also, you may not have a lot of monopoly to have features built to your specific taste. So please make sure you ask for required demos from the seller along with obtaining some free trials to test some features out
Suggestion #2: contact a local Microsoft Partner with an expertise in Business Central/NAV Technical areas and send them your requirements. Many partners have a product ready and can sell it to you at a much cheaper price
However, if you'd like the vendor portal to be built as per your needs and then the integration be built with BC/NAV, I think it can take upto 2-3 month of work but the source code is your IP so it becomes really easy to include changes/scale if the business grows
