Herculiner is very cheaply made in my opinion. Guess thats why they sell it for $26/gal at walmart.
As far as vinyl or rubber mat flooring, I have never seen any in the websites. Carpet kits is about all they sell.
I would think that the vinyl would sweat when it got warm, causing your floor to rust.
I have those thick rubbermaid floor mats in mine, and they work pretty well, cover almost all the floor boards.
Monstaliner is the way to go for an interior flooring. You can roll it on or spray it in with an undercoating gun. Its about 5 times as much as herculiner, but its 10 times the product.