Maybe im wrong but to me a doula is more for first timer than for second/3rd etc labours.
however, my first (and only) labour ended in a emcs and i was quite traumatised by it etc.. i have been wondering if i should take a doula this time around but the thing is i know that to the first complication i will have to have another cesarien.. so it it worth having a doula?
I'm a doula myself and have to say that the majority of my clients are 2nd or more timers! with the vast majority booking me as their previous (first) experience was traumatic in some respect, I am contacted in particularly by a lot of VBAC mums in exactly your position and have to say that I think VBACers benefit perhaps more than any other mums in terms of having a doula!
I tend to find that I am used much more in terms of antenatal contact by my VBAC clients, lots of questions/discussions and confidence building in their VBAC journey. Many doulas have a real passion for working with VBAC clients because it is so rewarding to be asked along with a mum (and dad!) on this special journey, secretly I have to confess that VBACs are my most favourite births too!
So, yes we are booked by first timers, but not always! we're definitely not only useful for first timers!
Have you contacted any local doulas to have a chat? usually we provide the initial chat for free, that way you'd get a really good idea of whether it would be a good idea to book one. Have a look on www.doula.org.uk to find doulas near you (I cover Essex/Cambs/Herts if that's any use). Also have you joined the Yahoo VBAC/HBAC discussion group? it's well worth joining if you haven't!