Using an Action Step in TestStand with LabVIEW adaptor to call the Vision Assistant Express VI, the results appear to always come back null (strings='' and numbers =0).
The Vision Assistant steps, in general, ignore the incoming Image, instead loading an image from a file. Then several common steps, like pattern match, set coordinate system, then the actual inspection: either a pattern match to verify a feature and/or an OCR. I choose to return NumberOfMatches from the pattern match, expecting to see 1, and I return the String from OCR steps, expecting to see the matched test. Those two items are returned by the ExpressVI as parameters Module Settings pane in TestStand, and I assign their values to locals so I can analyze or use them later. Instead of 1 match and some text, I see 0 and '' (Null) every time. Yet troubleshooting, I can see the image files newly saved to the hard-drive from the last execution and looking correct, and I can reopen the two steps in Vision Assistant (Configure VI...) and see it loads and analyzes the image exactly as it should within Vision Assistant and provides correct results. Assuming some corruption in TestStand I created a new sequence and created a new step from scratch, with same result.
I have used exactly this same method for previous projects, using LabVIEW 8.5 and Vision 8.5, whereas now I'm using LabVIEW 8.6.1 and Vision 8.6.4
I tried running a similar setup as you had and was not able to recreate your issue. For debugging this setup can you create a top-level LabVIEW VI that calls all of your LabVIEW VI's (in the same order as TestStand). Does the data transition through this top-level VI without problems? I'd like to see if the source of the problem is the transition between LabVIEW and TestStand or if it lies within LabVIEW.
One additional comment. Are you using the "Select Express VI" option or the normal "Browse for a VI" option in TestStand to select your Vision Assistant Express VI? If you are not using the "Select Express VI" option try using that.
Please let me know if this issue has been resolved.