If its not vegtan, it might be chrome tan or it might be a combination chrome/veg tan. A lot of chrome tan has a blue tinge in the centre of the leather.
If you have chrome tan, I think you might have a problem with getting a decent colour, unless you like black.
You could try to deglaze it with some lacquer thinners, bearing in mind that the thinners will dry out the leather* some, so a coat of pure neatsfoot oil (not the compound or light option) before or after dye would be a good idea.
*Also dry out your skin, so gloves are highly recommended, and a mask to keep the fumes out of your lungs.
Whichever way you decide to go, try it out on a sample first.