The Vineyard Theatre opens its 40th season with Sandra, an eerie solo show that dips into the murky waters of missing persons and false identities in order to demonstrate how physical disappearance can manifest itself in many forms. A friend will take off, a business will burn down, a spouse will depart, a house will grow bare, and a lover will become unrecognizable. It’s enough to drive a person to drink, and sure enough, given this title character’s unsteady relationship with alcohol, plenty of wine and liquor will also disappear. So much emptiness, but the result is a mostly fulfilling evening of theater.