Combine the bloody mary mix, red wine vinegar, and sugar. Stir until the sugar dissolves then chill until needed.
Salad
Peel the onion then thinly slice to get onion rings. Slice the cucumbers into thin even slices. Half the tomatoes, then thinly slice.
In a large bowl combine the onions, tomatoes, and cucumbers, then top with the dressing.
Chill for at least 2 hours stirring occasionally to make sure all the mixture is exposed to the dressing. Unlike most salads, this one you want to get slightly soft.