If you want amazing grip when mountain biking with a flat pedal, then you'll want to go with the Five Ten Freerider sneaker. The Stealth S1 rubber outsole is grippy and works good in both hot and cold weather. The Five Ten Freerider shoes also offer more protection for your foot, with a semi stiff sole, than Vans. The one downside is Five Tens are more expensive than your typical Vans shoes.
To make your mountain biking shoes more comfortable, I'd recommend taking out the insole and replacing them with a Sole Softec Response Insoles. If you have high arches, you will need arch support, which Sole Insoles do an absolutely great job at supporting your arches and your feet. They are more comfortable than the regular shoe insoles and will make a huge difference in keeping your feet feeling better and more stable while cranking down on the pedals while standing up to pedal or sitting down to pedal.