Something else to take into consideration is your iron levels and your haemaglobin level. Regardless of mode of delivery, if you lose a lot of blood, but your Hb was reasonable before birth, it's unlikely you would need a transfusion. If you're already anaemic, the chance of requiring a transfusion is higher if you have a large blood loss.
Given your choice to not receive blood/blood products there are certainly things that can be done to minimise the chance of a big bleed with a VBAC. Having an active third stage using either syntocinon/syntometrine/ergometrine will help your uterus contract down post delivery would probably be a recommendation the hospital would make. Risk factors for having a bleed post delivery include induction/augmentation with an oxytocin drip, a long labour where your uterus is working hard and gets tired and lazy after delivery.
While a VBAC has other risks involved such as uterine rupture etc, the risk is clearly so small otherwise the drs wouldn't have cleared you for a VBAC.
I think you need to think about what YOU want for your birth and then work with the doctors to provide acceptable alternatives to blood should the need arise.