Laura, I think the vaseline might work, but I wonder how well paperboard holds up to Vaseline. Perhaps you could rub it in and let it sit overnight and see if it seems like it will hold work.
Another possibility is lanolin. Lanolin has been used for hundreds of years to waterproof textiles.
Another thing you could do is cut a piece of oil cloth to fit the bottom and insert that into the heart so you'd have a good leakproof bottom. If you don't have oil cloth, some other plastic sheet would work, I think. Of course you'd still have the sides to seal.
Then perhaps pour once you have a thicker trace. I think a thin trace might be asking for trouble in this as a mold. But still keep the mold on or in another soap-safe tray or container for double protection while it sets up.
Everything looks like a soap mold to me now!
I've tried one of those before. It was nearly impossible to line. I might try it again sometime, but it made an awkwardly large bar of soap. However, I think it would be perfect to package some small soaps as a gift.
