The biggest issue I have seen with 8.5 Vista vs. XP is that if you leave Vista in the standard theme, the fonts have changed. I designed several front panels to have them be out of whack with XP. So if you are going to be using code across platforms, you need to keep in mind they will look different unless you use the XP theme in Vista, or customize your fonts to make sure they remain the same between the systems. The dialog font is a different size (13 on Vista vs. 11 on XP), and a different font (can't remember the difference). That was the big one I noticed.
8.5 over 7.1 is mostly going to be the learning curve to learn the new features. Overall, I have appreciated the changes, but there are some things (mostly development related) that I have seen run a little slower in 8.5 than in 7.1, but have not noticed any runtime issues as of yet. One big change between the versions is application building, which is more complex in 8+. I do appreciate the new features, though, but NIs project still hasn't rubbed me the right way yet.
NI doesn't support LV 7.1 with Vista. I have used it and haven't seen any problems, but that doesn't mean one won't pop up. If you're going to stay with 7.1, you better stay with XP. 8.5 is the first version NIs supports as Vista compatible. You will also have to use a relatively new set of device drivers, so if you have old hardware you are trying to use in your new system, make sure it is cimpatible with the latest drivers.
I have actually had more issues with other hardware drivers and software packages than I have with LabVIEW. TestStand is not yet supported in Vista, and i found out the hard way, one of the ways it is incompatible and had to move back to XP for devlopment.
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