Question: Could this 720nm IR filter from ebay be used for NDVI/infragram?

With this filter in front of your camera lens, the photos captured will include mostly near infrared light. There will be some visible light included because 720nm is really just very long wavelength red light, right at the edge of near infrared. There is no information about how good the filter is at blocking anything lower than 720nm, except their claim of "99%." To get these NIR photos, you would have to remove the internal IR cut filter from the camera or very long exposure times will be required.

A near infrared photo will not allow you to produce NDVI. NDVI is based on the difference between NIR reflectance from foliage and red reflectance from foliage. So you would need a second photo of the exact same scene taken with an unmodified camera. That's the best way to make NDVI images because it is the only way to get mostly pure NIR and mostly pure red photos.

Chris