So Wayne, if I might pick your brain, as you have the experience and I'm just starting to dabble...
LONG version: If I were to purchase a couple of these to go along with my SB-700, do you think it would be an acceptable combo in a two umbrella + hair light set up, or two umbrella + backdrop light? This might help me approach my desired speedlight studio set-up sooner rather than later.
My main curiosity is tied to whether I should actually try go with a matched set or if it's ok to mix and match flashes (whether or not the power differences would be drastic enough to create uneven lighting). Granted, I guess I'd be tweaking the power compensation depending on spacing and/or circumstances anyway, so this is probably a moot question. LONG RUN, I'd like these to be good enough to last until I can afford a better strobe kit, which would be a while, but still get some decent "studio" style of shots.
SHORT version: Is it ok to mix and match speedlights?
Sure, mixing is OK. Even mixing speedlights with larger studio lights is OK (but then you do see a recycle speed difference, we have to wait on the slowest one). An umbrella routinely should be "as close as possible" for softest light, but speedlights in umbrella are at higher power, and slower recycle.
Best advice I can offer: I think you'll surely want to plan on a light meter, to set the individual manual lights. We can eyeball one single light, but not three. The ratio between them is important, and metering lets you know what you're doing. You could wait to try without the meter first, but you will surely love the umbrellas, and may end up doing a fair amount of it, the family over the years, and the meter is sort of a life time investment. It's good to KNOW what's happening.
And maybe most important, the meter lets you easily duplicate results next time. See comments at
Why would I need a handheld light meter? ... near page bottom, at section "To illustrate the common way a hand-held meter would be used for multiple manual lights
". Also up a bit higher, see about tenth stops.
But the point here is, we set them all to different metered levels anyway, no advantage to be the same unit. The advantage of same light everywhere is that the control menus work the same way.
The VK750 is a good flash, and you'd probably want a radio trigger on one of them, and the other two can use S1 optical slave mode to trigger in sync from the flash of the other. I use a PC sync cord instead, to the fill light. Camera is on a tripod, and fill light is above camera, so the trailing cord is absolutely no issue. Optical slaves easily trigger all the others.
The Commander is sometimes knocked because it uses a very weak flash to communicate with the other lights, which does not have much range. But optical slaves are whole different world, their trigger signal is the full working power of the final flash, and indoors in a big living room, I have had absolutely zero problems with optical slaves, placed anywhere, without concern.
Speaking of Commander however, another alternative (not really any better, IMO), and it is spending your money, but you have the D7100 with commander. The SB-700 can use it. The VK750 cannot. But the Yongnuo YN-565EX can, for about $100, double price. It will do all the VK750 can, plus this, plus slightly more power. The VK750 and SB-700 should be about the same power, and the Yongnou a bit more, which would be of some advantage in a umbrella.
Note however, mixed with manual lights, we CANNOT use Commander in any fashion, particularly Not with optical slaves. Don't even think about it.
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You'd place the main light much closer of course, barely out of sight in picture, subject can reach out and touch the light stand pole. As close as possible (just out of sight). Close makes it be big (to the subject), and big makes it be soft. The Fill light has to be back by the camera (so the frontal lights umbrella does not block lens), but it is metered to be a stop or so down anyway. Still, you may want your strongest light here.
Keep the camera back about six feet from subject, or more (for proper perspective). But the main umbrella as close as possible, for soft and power.
Working at f/8 or f/5.6 is very nice for portraits. The f/2 stuff is nonsense.
For the differences noted, and because a D7100 can do it, I'd spend the extra $100 for a pair of YN-565EX. But the VK750 is a nice flash, comparable power to SB-700.
Reviews of both on my site.
But either system (commander or radio/slave) will do the job, and really, you don't want TTL with studio portraits anyway. Manual setup speed is slow, but you want to carefully set each light to the desired ratio, to KNOW what they are doing, to KNOW they are doing what you set them to do.
But still, the D7100 can do it all, it can't hurt to also have the flash capability. The Commander is extremely awkward for more than two lights, but is great for two in the quick setup TTL jobs, just take the picture and be gone. But critical portrait work will want studied manual flash. It is good to have both capabilities.
Also for lighting starters, see
45 degree Portrait Lighting Setup