What's the reason that it should not be given IV?
As near as I can figure, Vistaril IV or SC can cause neuro damage.
Pregnancy (especially early) or lactation; treatment of morning sickness during pregnancy or as sole agent for treatment of psychoses or depression. Hypersensitivity to drug. IV, SC, or intra-arterially. "
Low incidence at recommended dosages. Drowsiness, dryness of mouth, involuntary motor activity (rarely, tremors and convulsions), ECG abnormalities (e.g., alterations in T-waves), dizziness, urticaria, skin reactions, hypersensitivity. Worsening of porphyria. Marked discomfort, induration, and even gangrene at site of IM injection
Here's another site where apparently Vistaril was administered SC instead of IM, with pretty disastrous results:
"For several weeks afterward, the patient had hip pain and a lump at the injection site. The patient claimed she was unable to work. A neurologist two months later formed a diagnosis of cutaneous gluteal neuropathy
...the patient's injury could have been caused by a faulty subcutaneous rather than deep muscular injection
of the drug Vistaril, or that a nerve could have been hit by the needle tip due to inaccurate location of the injection site."
Hope that helps.