I haven't checked if there's an app for this, unless you have a keyboard I don't recall any way to do this, but you could for example connect remotely to your Venue 8 using something like TeamViewer and take the screenshot that way from your desktop instead, then just cropped out your local screen off the picture.
This didn't work for me before, but after installing the OTA update to Android 4.3 this morning,
it now works! Thanks for the advice.
Also make sure you hold Power and Volume Down long enough. I tried quickly pressing them at first, like on my phone, but it seems like you have to hold them down for a full second on the Venue 8.
You can press Power Button and the volume down to take screenshot on Venue 8. This method works for many Android devices. If you want to capture screenshot on computer, Acethinker Screen Grabber can help you a lot, free and works pretty well.