Couple things here.
It's well known that Vrabel is a Trumper. He may not be a flaming bigot but he's damn sure in alignment with the party of flaming bigotry.
Kraft doesn't have to be the direct source. Vrabel can get his true feelings out to Boston media fairly easily while holding his cards to Nashville media.
I always felt Vrabel wanted GM level roster control after JRob was fired. And since he's shown he can be a pissy queen at times, it's totally realistic to believe he's spewing this flaming bigotry against RC out of spite.