There's no ceremony or anything that makes one a believer of a particular god. Many people worship multiple gods as well. Technically, even turning Heinz into a Vida race wouldn't make him a Vida believer. It would be up to him to declare and act on his beliefs. Though if Heinz wanted to show sincerity it would be a decent method because it can't be reversed (there's no known way to convert someone from a Vida race to a "normal" race without reincarnation). Also, it would probably be accurate to say that Van doesn't actually care about such things. He likes to have friends and allies and anyone who is evil (by his judgement) or a threat to his happiness is terminated - that's about it. If Van knew how to kill Alda he would have done so already. In Van's mind, anyone who has attacked Darcia or ordered someone to attack Darcia is on his must kill list - if they've done it once then they can do it again. He's specifically stated this. It would be cruel of Darcia to ask because she's fully aware of just how much Van has suffered. Van literally became ill at the very idea of not forgiving those like Riley and Gordon even after he'd killed them.