I dismissed the idea previously, but after today I am leaning more and more towards this being a possibility. I feel like he's the only one out of the four that would have all the others (plus Trina) cover/lie for them.
Something that stood out to me in the parent's last interview they did together was when Vernal said, "I don't know what happened to my son, I left him with a trusted and respected adult, and that's the last time I saw him." That seemed like a really odd way to refer to the GGP. IMO he added unnecessary qualifiers and at the same time was distancing himself from the GGP by not saying his name or his relation to the family.
Maybe Vernal helped cover up an accident involving GGP and that has put him in a tough position because he'll end up in trouble too (tampering, obstruction, etc.) if he reveals the truth at this point? That seems logical to me today, but I'll probably think something else tomorrow. Whoever said this case is a circus hit the nail on the head.