i asked my vetwhen i should breed my bullmstiff bitch.she had her first season and after had a phantom and filled with alot of milk.i said i was worried about this and phoned the vet,she said i should if i was breeding her to get her lined at next season and this would stop

Actually mating a bitch will not stop phantom pregnancies, she will just have a real preganncy, and may then contiue to have phantom preganancies or not.

Just because she had one last time doesn't mean she will have one next time, though it is more likely.

You should only breed from your bitch if you realy want her to have puppies for a good reason.  Has she done wll at shows?  Is she a real top class animal whose genes will be of benefit to her breed.  Most importatnly has she been xrayed and her hips been assessed under the BVA?KC hip Scoring Scheme and come back wityh a low score.

Have you the facilities, time and temeprament to rear a litter and then provide life long assistance to the new puppy owners, and possibly have to rehome any of them if owners circumstances change.

These are large powerful dogs who can so easily be spoiled by the wrong upbringing, you could then have an unruly youngster to contend with, one with joint problems through incorrect rearing etc.  Breeding can also be heartbreaking as sometimes it doesn't all end happily ever after.

The bitch wshould be a minimum of two years old before being bred from, and iwth such a large breed nearer 3 would be ideal.

Having a litter won't stop her having phantom pregnancies afterwards. The only way to guarantee to stop phantom pregnancies is to have her spayed. I would strongly recommend you get a copy of "The Book of the Bitch" by J.M. Evans and Kay White - you will get far better, more accurate information there than from your vet, who seems to know very little about dog breeding.