Can I add vanilla to pastry cream after its cooled?
If your pastry cream can handle being stirred (most do and as you will be including whipped cream anyway), you should be fine. Stirring will soften your pastry cream a bit, but not make it completely liquid.

You can even add the vanilla to your cream, whip it together and need not worry about stirring well enough or uneven distribution of liquid in your pastry cream.