I think we all know the difference between the sort of badly put together books pumped out by a vanity press and the sort of book published by a good mainstream publisher like R&L.
I've seen all sorts of books published by vanity presses. The majority of them are bad, but good ones get published by those publishers too. Quality isn't an indicator of a vanity press.
The OP wrote:
This pretty clearly implies that R&L is a vanity press or lacks credibility when it most certainly is not and does not.
Judging from the other comments here, you're the only person getting this from the OP's post.
I have no idea why the OP would believe that working with a publisher like R&L is a waste of time. They are a highly respected publisher focusing largely on books for the academic and popular non-fiction market.
The OP was asking a question, not stating an opinion. You're adding all sorts of excess value to his comment.
One of the primary advantages of working with them is that they read unsolicited manuscripts. So I'm not sure why the OP is even bothering with an agent.
To play devil's avocado here (yep, I know), perhaps you should look at this from the other side: if the OP has an agent, why are they bothering with R&L?
To answer your question, writers with representation routinely attract higher advances and sell more foreign and subsidiary rights, regardless of which publishers they sign with.
An agent is a far better idea if you're aiming for a far larger publisher. But there's nothing particularly wrong with a midlist company like R&L.
A publisher can't be a "midlist company". Only authors can be midlist, because the midlist exists between a publisher's frontlist and backlist.
It's good to get an agent if you want to make a living as a writer, or if you want to get the best possible deal that you can, or if you want to sell as many rights as you possibly can. It has nothing to do with the size of the publisher you want to sign with, or where you sit in relation to your publishers' lists.
And none of this has anything to do with the OP's original question, so shall we get back on topic now?