you could try giving food rewards when putting on the harness, but only for good behaviour (you dont want to re-inforce bad behaviour), or start doing some simple obedience work with her eg stay (down stay is a good one for dominance issues).

please try not to take offence but your vet sounds a bit paranoid about the small dog/big dog thing (i know it could happen)

our chihuahua boy only gets obedience trained on a collar (correction chain or martingale, but with out the corrections) and is on a harness the rest of the time or naked so to speak

good luck with getting desi lu used to the harness.