Go to: https://www.nhdeeds.org/

Click on your county, accept the disclaimer at the bottom of the page, click on AVA search, enter your neighbor's last name and first name, then hit enter or click submit.

Their mortgage company will be listed along with their name(s) as the lien holder. Their mortgage number will probably be listed as well.

Then you can do a search for contact information on that mortgage co/bank and get in touch with them via phone or email. Tell them about the small vent fire and I'll guarantee they'll send out an inspector to conduct a through safety inspection on their half of the property.

If they have a trampoline, above/below ground pool, fireplace, wood burning stove, or other hazzards, they will need to be within safety compliance.