I used vasoline more as a temporary mount. I siliconed a foam insert cutout in the bottom of the hull towards the front. Once the foam insert cured, I heateda jar of vaseline in boiling water. Once it was almost liquid I poured some in the foam insert, set my transducer in and then filled the rest of the space with vaseline. Once it solidified, it made a great bond. I store my kayak upright so hopefully it won't give me a problem. If I have any problems with it, I will probably permanently mount it.
The reason I didn't mount it permanently as Tex mentioned was because I read some posts about air bubbles under the adhesive, so I wanted to experiment first with the vaseline to see how well that worked. It hasn't given me any problems yet.
Good luck. John