Licensing and insurance, verified — not just claimed
Any plumber can say they're licensed and insured. Ask for the actual license number and check it against Washington L&I directly, and ask for a certificate of insurance naming your company as additional insured if your agreements require it. A vendor who's slow or cagey about producing either one is telling you something.
Do they respect your NTE limit, or treat it as a suggestion?
This is the one that costs money quietly. A vendor who proceeds past your not-to-exceed limit without calling first is a vendor who's decided their convenience matters more than your authorization process. Ask directly how they handle work that exceeds the NTE before you hand them a job, and watch how confidently they answer.
Documentation you can actually use later
Photos and a real written description of the job, not a one-line invoice memo. When an owner asks what happened at a property six months later, or a tenant disputes what was done, the vendor's documentation is what settles it. If a vendor doesn't photograph jobs as standard practice, that's worth asking about before you need it.
How they talk to your tenants
A vendor who negotiates scope or pricing directly with a tenant on-site is a liability, full stop — it puts your tenant in an unfair position and puts unauthorized charges on your desk. The right answer to "who do you talk pricing with" is always your office, never the person who happens to be home when the truck shows up.
Onboarding effort — a real signal, not busywork
A vendor willing to do a real setup call — authorization contacts, billing contacts, your portal, your packet — before the first job is a vendor planning to be a long-term fit, not a one-off. The five minutes it takes upfront is usually a good predictor of how the relationship goes after.