Hi James, you're really asking about more than one product here - a retarder to slow drying, a blending medium and a matt medium.
Matt medium: I'd suggest forgetting about these entirely as any medium you add to acrylics will increase their transparency somewhat and high opacity is one of the core benefits of hobby paints like Vallejo, plus many are not truly matt anyway. If you have specific problems with shine in one or two colours I would:
look for alternative paints that will solve the problem - this is the best long-term solution;
or, add in small quantities of a matting agent - Tamiya's Flat Base is probably the easier to find.
You can also develop a reliable overcoating procedure so you can fix any problems at the end of the painting process, because you then should go back in and add gloss to leather, skin and painted surfaces etc. I think this is the least good of your options.
Retarder: in general I generally recommend hobby painters avoid these for a number of reasons, primarily because if you use too much, which is very easy, you can end up with a paint layer that won't dry properly and can lift. They can also increase gloss.
Blending medium: there are a number of products in this line that are worth trying. I don't use any of them but most of the makers of acrylic paint for artists will stock something along these lines, if you have a nearby Dick Blick store I'd suggest popping in and taking a look. If you want a specific recommendation from a Vallejo user Jim Flagstad has an article online somewhere (sorry, don't know the link but someone else might) where he suggests using W&N's Watercolour Blending Medium that I think also slows drying a little as a bonus.
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If you're having trouble with the paint drying on the palette too quickly - hardly surprising given how dry it must be in Nevada - I would strongly suggest buying or making a stay-wet palette if you don't already have one. That way at least the paint on the palette will take care of itself so you don't have to continually fuss with it and can concentrate on getting it on the model.
As far as blending problems go, another tip well worth trying is to use the very largest brush you can to do any job. Many of us paint figures like you average 54 with nothing above a #1, which is a mistake, as larger areas can easily be started with a good #4, or even bigger, if they have a good point. These larger brushes hold more paint, so they dry out less quickly, as well as physically covering more area with each stroke, speeding up the painting process. You might be surprised just how much of a difference it can make, just takes a bit of practice to get used to a 'large' brush on a small area.
Einion