Okay, I went though a similar ridiculous delay when I moved money a big chuck of money from Edward Jones to Vanguard. I'm fairly sure the delay was really on the Edward Jones end, but they blamed Vanguard (because of course they did.) The money showed up eventually.
You are wasting your time on the phone, most likely. What you need is written correspondence that makes it clear you are a well organized professional. Type up the dates, missing amounts etc. as well as the dates you spoke to people and the names of the people you spoke with. Remind them that trust is the foundation of the financial services industry, and that you expect them to figure out what has happened to this money and rectify the situation by updating the account. Tell them in the letter that they have 30 days to correct the situation and provide evidence that they have acted in good faith, and that your next step, failing resolution of the issue by them, is to contact the FCA with a formal complaint (or whatever govt agency will fine them . . . try to figure it out and name the right agencies.) Keep it short, to the point, polite, and professional, and send it via Federal Express with signature upon receipt required. Send basically the same letter to each place, but don't let them know that you are holding the other accountable: put the blame squarely on Omni in the letter to them (you claim to have sent the money via ACH, but Vanguard has no record of it, so it is up to you, Omni, to track this down), and put the blame squarely on Vanguard in the letter to them (I have confirmed with Omni that they sent this money on this date, Vanguard, so it is your responsibility to figure out why it's not in my account already.) I'm not kidding.
Then don't think about it for a month. Seriously. See what happens.
If $400K owned by a collection of people really went missing, then that is a class action lawsuit with punitive damages in the making. Nonetheless, they'll all say that they don't know anything about it if you don't send them something trackable in writing. Good luck to you!