Greg knows perfectly well how the rest of the school sees him and Vincent. Great lumps of muscle, with no brains to speak of.
Even Draco is guilty of this, and he spends more time with them than anyone. They don't object, because Lucius pays them well to protect his son. Why throw away a source of ready money?
Nobody really calls them friend. Nobody knows them, not the way they know each other.
And it is only when Greg is alone with Vincent that he can give and receive the respect and, yes, love, he knows they both deserve.