A trick which has worked for me in the past is to open a second instance of Vegas alongside the faulty one, copy and paste the entire timeline into the new file, save and render this. Caution: you may have to copy and paste track movement settings separately.
Thank you all for your thoughts which will help a lot; Jon – the test is within Windows 7 and linked to the card: it just proves that windows can output 5.1 to my amp.