Yermin shouldn't DH against a righty. Maybe you throw him in there against a soft tossing lefty, but he's totally unable to handle ML pitching right now. If you have to, send him down to AAA to figure it out, cause having him hit .150 in the majors isn't helping anyone.
Lamb should DH vs. righties as much as possible, but stop putting him in the 2 spot. He's a total liability on defense and really shouldn't be put in OF, especially if you plan on playing Goodwin in CF. The combination of Goodwin, Vaughn, and Lamb might be the worst defensive OF in baseball.
Play Engel as much as possible. We need him to anchor the defense in CF and he's hitting well enough against RHP that he shouldn't strictly be a platoon player. Especially when your other options in CF are Goodwin and Leury.
Against RHP, I'd go Lamb at DH, Goodwin in LF, Engel in CF, and Leury in RF. You can even give Collins ABs at DH and have Lamb in LF if we don't have a fly ball heavy starter on the mound.
Against LHP, Vaughn needs to be in the top 5 of the lineup. I don't care if it's 2, 3, or even 5, he belongs in there against lefties. He's one of our only guys who's had success against high velocity this year.