If you are looking to buy a cheaper to middle of the range Mountain Bike in Vung Tau, I found a good shop located at:
80 Ba Cu Street, Vung Tau - in the main shopping district. walking distance from front beach.
Tel: 091.3758822 Mr Hoa.
The frontage is all kiddies bikes so it looks like a toy shop fm the street, but if you go inside, he has a good range of Mountain Bikes hanging along the walls. They have Shimano gear etc so decent bikes. He also has a great range of accessories like, helmets, bells, horns, lights locks etc so a good score.
There is a high end bike shop at 128 or 148 Tran Phu Street. His Mt Bike start at 20M and he mainly has and services road racers with high end gear. no english though. It's a small shop with maybe one or 2 bikes inside so not that obvious. It's about 300-400m along Tran Phu on the left after you turn left into this street off Le Loi street - down near the fishermans village market.