First, congrats on your blooming hydrangea! Now is the time to cut and harvest those blooms making the cut just above a node or joint at any length depending on the size of your vase or how you will use the blooms. Then just let winter put the hydrangea into dormancy. In early spring when you see forsythia in bloom or the buds on your hydrangea swelling you can do a proper pruning of the shrub by shortening the branches that are long and removing any weak or small branches. When it comes to the Pee Gee hydrangeas like Vanilla Strawberry, the less branches you leave the bigger the blooms. Remember you don't have to prune at all if you want a more casual shrub. Pruning is to tidy up the plant by having less side branches.  Those big blooms  do make great autumn displays however so get snippy and enjoy your harvest. Keep growing, Marianne Binetti P.S. I do have some hydrangea pruning videos on You Tube.