With the "out of phase" question, I would simply make sure that I was running the latency test at the beginning of the SPEC guide to make sure my FFT size in SPEC matched what the Plogue settings are. In a future build of SPEC, we're going to eliminate that step entirely but, for now, it's needed.
As for vocals in the rears, it's going to be both source-dependent and how meticulous you really want to be while live monitoring, if you're live monitoring at all. We aren't a bunch of studio mixers sitting at a mixing board with multitracks here. We start off with limitations right off the bat. It doesn't mean we give up and say it's not worth doing, although there certainly is a part of surround community which feels that way. The fact that we can get to where we can get to is amazing, with credit to be given to both Zeerround and those who paved the way with earlier stereo-to-surround methods. If the vocal is perfectly centered in the stereo, you stand a much better chance of centering it in the conversion. If the vocal tends to expand to the entire soundfield, you have a bigger chance of hearing it in the rears. You can be aggressive with your separation in the rears with all the SPEC variants, but also need to remember that, as you remove vocal, you're also removing other parts of the mix. You may be left with a lot of partial instrumentation and vocal in your rears which may result in some audible distorted sounds in your final mix if you get too aggressive. You also might leave just enough in there that it's masked by the rest of the mix and you can really only hear it if you put one ear extremely close to the speaker. The balance between separation and soundfield is one best left to the person doing the conversion. ArcTan, without the Slice rear blend, may offer you a good balance. There are other methods out there on the net which are completely soundfield-dependent, but aren't going to work well with denser mixes. Lastly, we all have drastically different speaker and listening setups. What you hear on your speakers may be different than what I hear on mine. That fact alone explains a good 50% of the differences of opinion that go on on every surround-oriented group out there.
I had the good fortune of having you share some of the work you're referring to with me. It was music that is now 40 years old. There was vocal in one of the rear channels but, when sitting in the sweet spot, the vocal appeared entirely centered. That may be the best you can do with that specific album. Someone doesn't like it? They can move on, I say.
I'd also say that there's plenty of retail mixes, quad transfers, etc., out there guilty of rear artifacts, partial vocal, etc., and we love them.
In the end, do you like what you hear? Would you feel proud sharing it with a friend? That's what matters.