My bitch has had 2 Voren injections to treat allergies. She last had one late July (which would have worn off by mid August) and has been mated yesterday (a whoops mating with our stud dog). Is this likely to cause any problems with the pregnancy or with the pups? My vet was unsure of a withdrawal time between steroid use and mating so any information would be great.

I would seriously consider taking her to the vets and having the possible pregnancy stopped, as your bitch has an allergy she is very likely to pass this onto her off spring :( 

The fact she is being treated with Steriod at all for allergies then the pregnancy should not continue.   Most allergies dog or humans rarely require steriod unless quite serious.

Contra-indications for voren suspension
corticosteriods are not recommended for use in pregnant animals, administration in early pregnancy is know to have caused foetal abnormalities in laboratory animals . use in late pregnancy may cause early parturition or abortion
I would go back to the vet and have her injected to prevent any pups.

As the other peeps have said with her problems and steriod treatment I would recommend getting the injection done to terminate the pregnacy.