From the research that I have done and examples I have looked at, it looks like the OBX segment of an HL7v2 message can be either a lab or a vital. Is there any universal way of distinguishing what it is?
I have looked up some stuff but don't see anything that identifies the difference between the two.
OBX segments can represent a variety of things, not just strictly a lab result or vital sign. For example, OBXs can contain a report or document as an embedded, base64-encoded PDF.
For cases where something discrete is being measured, like a vital sign, OBX-3 is the observation ID to identify what value/entity is being measured. The only universal way to know what that entity is is to utilize a standard code set, such as LOINC codes. If the system generating the HL7 message with the OBX segment is including such a code, then you could interpret what kind of result it is based off of that.