We use "Trans Prep" at work (the company I work at makes industrial gearboxes).
It's made by Life Automotive Products Inc, Memphis, TN. It's about the consistency of grease but WILL thouroughly dissolve at temperature. Good idea with the Vaseline though...
BTW: you can also mix the trans prep with
oil to make it thinner for putting on seals, etc. as a pre-lube.