Warmth, tingling, yawning, sighing, changes in breathing, a slower pulse or a different HRV reading can all occur for many reasons. They may be meaningful experiences or measurements, but none is a standalone proof that a particular vagal target was engaged.
HRV is especially easy to over-interpret because respiration, posture, recording length, recent activity and other factors influence it. It is useful in research when methods are controlled, but it should not be turned into a consumer score for vagus strength, damage, successful stimulation or treatment response.
Electrical VNS has its own evidence category because it uses a defined device, stimulation site and electrical parameters. Even there, evidence belongs to the studied protocol and population; it cannot simply be transferred to a different device, behavioural practice or wellness claim.