In my opinion, it's a rare occasion when, excluding the case of a white or cream sweater vest, you come across a sweater vest that looks better with a blue shirt, than a white shirt. This sweater vest is one of the rare cases. A white shirt looks fine with it. A white shirt is almost never offensive. But, the standard blue OCBD works better. It just does. It's obviously due to the gray, blue, purple and turquoise colors found on the sweater. All cool, very dark.
The white OCBD works fine, but the blue OCBD compliments this fair isle sweater vest so naturally. The compliment of the blue OCBD slides right into the hue and overall tone so seamlessly. The choice is obvious. It is as obvious as the choice of going up hill, or down hill. There's really only one choice, if you have the choice.
I picked up this Polo Ralph Lauren (90s or early 2000s) fair isle sweater vest for just $6.99 at a thrift store. The shirt is a Brooks Brothers blue OCBD and the trousers are Lands' End.