You have to book a tour with Oxalis which is a monopoly tour provider in Phong Nha. You cannot visit the cave otherwise. This trip is 2 days trek, they provide most of the equipment and food which is amazingly delicious. It's one of the best experiences in Vietnam!
In my opinion most of the trips offered are a rip off, guides are often bored and not interested in having a good time with tourists. Except for that, the cave and activites are fun - there's a zipline, kayaks, mud baths and swimming in complete darkness.
Oxalis has a monopoly for cave tours in Phong Nha and thank god - they know what they're doing, they prepare tourists of what's coming, what to bring, what to expect. The Hang Va Cave was great, a bit demanding even for medium trained people but so rewarding!