My b&s mower ran fine. I replaced blade, which meant mower was on it's side for a while. Now when I pull the cord I get about 1/3 of a pull and it jerks the cord back. A guy at Lowe's said it may be a vacuum block, but he did not know how to fix. What's the best way to fix it ?

It's the oil! If it was on its side for any length of time you have oil all over the wrong end of the engine. Don't force the cord! Pull the spark plug, and drain any oil that comes out.
Might be a good time to change the oil anyway remove the drain plug and remove all the crankcase oil, there may be little that comes out because it has migrated past the piston and now is in the combustion area. Spark plug removed the cord should pull easily, this will force oil the oil trapped in the top end. Its going to fly out of there so place a rag over the hole. You will need a new spark plug that one is going to look oily, easier just to replace it.