Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to reconnect with a former vendor, use this factual base: remind the person where you worked together, when, and what current topic makes the contact relevant.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Make one modest request and give enough information for them to decline or redirect it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not imply endorsement, exploit a former hierarchy, or send confidential material from the old relationship. Use official business, privacy, conflict, and procurement rules when commercial or regulated information is involved. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.