Depends on what you want to do.... VGA is, as far as I know, an analog interface so putting a logic analyzer on the actual VGA signals probably won't do much for you. If, on the other hand you want to monitor what goes on on "the other side" of the DAC then a fast logic analyzer would be the key. But then it's the number of bits in the interface bus to the DAC and not the number of pins in the VGA connector that stipulates the number of channels you need....

/Henrik.