Could taking anti-histamines cause vaginal dryness? can this condition be treated

Yes, it can. : Antihistamines can cause mucosal dryness including vaginal dryness. You can half your current dose of antihistamine to see if it continues to give you the anti-histamine benefit without giving you the side effect since the effect is dose dependant.

Yes: Antihistamines traditionally can dry any mucous membrane, including the vaginal tissue. Obviously stopping the antihistamine can help, but if that isn't reasonable, using some of the over the counter lubricants (Replens, Lubrin, etc.) can help.