That all msut have been a big shock for you, particularly the potential for a massive change in your birth plan.

With the low lying placenta, I understand they leave it until 32 weeks as by then most have moved nicely up and out of the way. I think if you had repeated scans between now and then you'd probably get a bit disheartened if it wasn't moving a little each time. I know if it was me I'd leave it until 32 weeks and then see where we are at. My friend whose girl is 3 weeks older than our boy was found to have a low placenta at 20 wks and had the same coversation with the dr re possible c section. Her placenta, like most, moved up and out o the way by 32 weeks and she went on to have a great birth - laboured at home to 9cm, no drugs, natural delivery. Oh, I've found some stats re placenta praevia. Rate is one in every 200 pregnancies, and only 2-5% of women who were found to have a low placenta at 20wks went on to develop placenta praevia bad enough to need c section. Sounds as though you have vasa praevia not placenta praevia, is that right? Or do you have both?

From the eMedicine site (and other medical sites):
Velamentous insertion: 1% of singletons, 15% of twins, common in triplets. Risk of haemorrhage if vessels torn when membranes rupture.
Vasa praevia: where the vessels are in front of the presenting part of the baby (kinda like having the placenta over the cervix, here the large vessels are situated over the cervix and adhered to it), rate of 1 per 2000-3000 deliveries. Vessels not likely to move out of the way but can do so. Monitoring during labour can show some of the indications of distress or vessel rupture. Doesn't say how often the vessels do rupture, but that event has a terribly high mortality rate for the baby (50-75%). Suggests c section is the best way for safe delivery due to these risks. On the positive side, where this is diagnosed early and delivery by c section, baby survival rate is 97% - unfortunately where it isn't discovered until birth, survival rate is only 48%. Oh gosh, that's scary!

I find lots of the website info a bit scary, perhaps you could return to your Dr and go through it all a bit more slowly? Or perhaps see a specialist in VP and VCI? I'm thinking with all this scary info that I'd be asking for a specialist US to be done (I know in my area there's an incredibly skilled OB who only does US and antenatal diagnosis work, she's fantastic) to check where exactly the vessels are, as it seems there is a big difference between having just VCI and having both VCI and VP.

There is a US vasa pravia website for the international VP foundation, perhaps it is worth looking at that if you haven't already?

I hope things turn out well for you, I'm wishing you all the best, please stay in touch and let us know how you get on with further investigations and also with the eventual birth of your lovely baby.