It's difficult to determine. He should be able to estimate his power when considering Galu could estimate how strong SSJ Gotenks would be despite having yet to see him, though Vegetto was also surprised by the strength he'd been provided, so whether or not he'd be able to calculate perfectly is arguable. In any case, it wouldn't affect anything, since if you're trying to create a link between this and Baby Arc scaling, Baby wouldn't be able to access and feel what Vegetto felt, thus making anything beyond SSJ Vegetto a non-factor in his "greatest Saiyan power" statement.
As Supreme said, the evidence for SS3 Vegetto being on SS4 tier is fairly vague when it never points to a specific form (only showing Base Vegetto of all things), never specifies which SS4 it's referring to and its Chinese origin makes it dubious as to who gave permission to make the entry. It can be considered logical to assume this via scaling, though it shouldn't really be treat as definitive evidence.