I've kept a list of vendors like these from managing my properties & being a realtor.

You find local handymen and vendors to fix them quickly.

So here is my question, I know a reputable firm that provides this service but how can I set up the 'business partnership' where they are my chosen vendor for this service without my clients trying to go behind my back to deal with them directly, thereby kicking me out the middle.

I've found a realtor's network of vendors is a great resource versus the agent actually doing the fixing and flipping.

Perform the same task in each app to ensure a fair comparison.Load a property with pictures and details.Market that property.See what your marketing looks like from the public's perspective.Submit a fake application to see how easy the process is.Run a credit/screening report on yourself.Enter a maintenance request, assign a vendor, and attach a fake invoice.Enter charges to the tenant's ledger.Enter recurring charges and automatic late fees.Sign documents electronically.Run owner reports.After testing a few apps, one should clearly stand out.

I would be focusing on cleaning up bookkeeping, communication portal with client, vendor list management, payment processing, etc. 

If you have all your vendors ready to go ahead of time and ways for them to get into the home if the tenant isn’t there you should be ok. 

For example, a plumbing issue comes up, and Appfolio automatically creates an invoice from the property manager to me for the plumbing services provided by a vendor. 

You want a property manager who is based in that community, who has established relationships with vendors, judges, inspectors....etc.     

Either the property owner leased the equipment and the UCC was intended to notify the world that a third party owns it (no, that vendor cannot now or later lien the subject real property) OR the property owner purchased the equipment and that 2022 lien is about to get wiped by the foreclosure of a first priority mortgage.