|Philipp Franz von Siebold and Joseph Gerhard Zuccarini|
(Flora Japonica, Sectio Prima (Tafelband).) [Public domain]
Hydrangea cuttings will propagate in damp vermiculite, coarse sand or other sterile medium. A terrarium-like environment will give your cuttings the best chance to sprout roots. But you don't want to stick your hydrangea cuttings down a narrow-necked bottle terrarium and hope to fish them out again.
Much like quarantining plants for use in a terrarium, you'll want to be able to easily retrieve the plant at a later date. Consider purchasing a wardian case with a hinged door, or you can build a terrarium environment by tenting your potted cutting with a plastic bag. Check out hydrangeashydrangeas.com for some great tips and photos.