Answer

Is there supervision by the Ministry of Health regarding its being vegan? If so, then there is room to rely on that it is vegan, but you still need to check tithes and priestly dues, orlah, worms, wine used for idolatrous libation, and ordinary non-Jewish wine, etc. It’s hard to rely on his own testimony, both because sometimes he doesn’t know and because he has a vested interest. When there is supervision, one can rely on what he says about the situation at a time when no supervisor is present, because it is something that is liable to be discovered, and he is afraid for his livelihood.