I have a hibiscus that was growing with primarily a single stem so I decided to make into a tree form hibiscus. I keep any suckers pruned back and also keep the trunk clean of new branches. So far, so good. Sometimes, however, hydrangea trees are shrubs grafted onto a hardier trunk. But, not always. If it looks like your Vanilla Strawberry hydrangea has a strong leader, you might give it a try. And, let me know what happens.
Thanks for writing!