So this lady is just "sick and tired of her dog"?!?!?!? well, i got Taylor at 6 weeks which was way too young but that's another issue. So she had her first injection at 6 weeks, again at 10 weeks and then at 14 weeks. I had one more after that simply because she had such a rough start and the vet wanted to make sure she was properly covered. A normal dog has their first injection at about 10 to 12 weeks (I stand to be corrected there) and every 4 weeks after that. there are normally 3...well in South Africa that's how ours work.
Go and see for yourself what this woman's other dogs are like and how well cared for they are...and keep a look out for breeding. I dont understand wht she would buy a Yorkie for breeding and then get rid of it because she is "tired of too many dogs".
They can go into heat any time after 6 months...it depends on the dog most the time. Taylor is almost 7 months now and she hasnt come into heat yet but I'm expecting it any day. And it is true that they should only breed after 2 or more heats. the more mature the dog the better.
If you take her to the vet he should be able to tell you how old the dog is...but she must have a record otherwise the vet can't give you the benefit of the doubt and i presume he will want to re-innoculate her to be safe rather than sorry.
otherwise put the word out on this site and you will find many brilliant breeders who'd credentials many others will vouch for. You have to pity an innocent animal who's owners just get tired of it and flog it off to someone else. So for the dogs best interests, i hope it all works out for you and her