Hi Ceri,
Cycling is a great way to see Vietnam, however most of the people on this branch are independent cycle tourists. You may have more luck posting on the Vietnam branch.
Having said that, have you thought about a bit of DiY? I have googled a couple of these tours and they seem very expensive to me - upwards o $3000 for just over two weeks. To put that in context, I need about $10 a day when cycling in VN. That means an independent cyclist can ride a 2 month tour H>S for $600. That leaves a lot of money for beer when you arrive in Hanoi!
There are plenty of people here who can give advice, but realise that it is a bit short notice now.
So, if DiY'ing seems a daunting prospect and you only want an organised tour, go for it, because even a bit of cycling is better than no cycling.