To improve data collection in the field, we want to enable Barcode / QR Scanning / OCR where appropriate rather than it having to be built into the App.  We've started to look at Virtual Keyboards on Android devices. Do any of you have any experience / recommendations for this type of approach?

Try using speech to text, which works quite well with Android devices. Barcodes and QR work well too. OCR has accuracy issues.

I've implemented scanning apps numerous times, and have difficulty coming up with a realistic scenario where it is more convenient, accurate or appropriate to manually enter a code when there is a bar/QR code to scan. Inevitably, your data quality will suffer but I can't see how your productivity wouldn't also suffer, unless what you are scanning is only a few characters in length.

For OCR, that might be a different story, depending on the scenario. If it is a single page or small batch you need to scan for OCR, or if this is a field location and you only have a portable hand OCR scanner, then that might be more trouble than it's worth.