As a disclaimer, I’m not a professional rescue rigging instructor. I’m on a sar team and do the work and often I’ll need to understand something myself more clearly. Breaking it down step by step I can share that process with others, and maybe you can benefit from it too.
Before you put your life or anyone else’s life on a rope system you should (seriously) get professional training, including about any topic I’m showing you.
Basically the single vt prusik on a twin tension system can replace the tandem prusik belay (TPB). The TPB seems great except that it’s nearly impossible to maintain that level of focus for an entire operation. Slack and user error get introduced early. You can watch professional rescuers in training screwing up the TPB all day long. For years SAR teams have been comparing the twin tension with TPB model and it’s not hard to convince folks to move to twin tensioned vt setups.
Studies and videos show a critical problem of the TPB: long catches. Falling 8′ before the prusik arrests your drop can be the difference between you dropping into a ledge and decking, not to mention the dynamic load that is created and transferred. You can run a single asymmetrical (shown below) vt instead of two normal prusiks because with a normal prusik only half of the six loops are really holding the load. So two normal prusiks are needed to actually create six grabs. With an asymmetrical vt you get all but two loops to grab, coupled with better heat tolerance.
In general, a better is a twin tension system with a single vt prusik on both the main and the belay.
A couple of other notes from my own research.
The VT Prusik is a brand name of Blue Water Ropes and cost ~$30 a piece. It is not acceptable to use some other sewn-eyelet cords and imagine them as VT’s. The studies you’ll read about are unique to Blue Water Ropes’ product so unless you know what you’re doing, stick with Blue Water. It’s about the material, strength, grip, and temperature rating: switching to random sewn eyelet cords does not give you that.
Conveniently there are two colors, tan and black. The 7mm is only available in tan, but the 8mm comes in tan and black. Make your life easier and buy 7mm tans and 8mm blacks so you can grab one and know what it fits.
The 8mm VT fits 9mm ropes and above, and is basket rated at 29.5kn.
The 7mm VT fits 8mm-9.5mm ropes, and is basket rated at 22.6kn.
Four VTs on your rack now cost you $120. But consider that the end-to-end rating of an 8mm is 20kn, allowing it to be used as an extender, basketed for quickdraw, etc.