Yikes! That is one brown rhododendron and the crispy look is usually a sign of drought or root damage from a mole or move, plus lack of water. But if the vinegar is sprayed onto the roots or foliage of a rhododendron you would certainly damage the plant. If the vinegar was only used on the lawn several feet away from the rhododendron roots then I doubt it would cause this much damage. One other cause of death could be a bacterial root rot - usually this happens after a warm wet spell, but it could take a few months for the damage to kill the entire shrub. I suggest you pull up the shrub and check the rootball. If it is very dry than blame lack of water, if it is slimy or soft and rotten it is the root rot. I suggest you try a different type of shrub in this spot that would be resistant to rot. A hydrangea could work, or if you want another evergreen consider a Pieris japonica. Keep growing, Marianne Binetti