Your eggplant can get mushy for a few reasons. Be sure to not use too much oil, and make sure to salt your eggplant to help draw out its natural moisture. Each slice of eggplant only needs a little bit of oil on both sides. If you do not bake your eggplant before assembling, or if you do not bake for long enough, there might still be moisture in the eggplant. This can result in a mushy vegan eggplant parmesan.