can confirm that I've seen this problem reported many times before ... capturing to the VO tool seems to be problematic with the Decklink inputs.
a search on "VO TOOL" at the Blackmagic forum on the COW revals much, including:
"To enable Voice Over Tool, you need to follow a similar procedure to DV, and use the following settings. Use the Displays menu (page 13 of the DeckLink Macintosh v2 manual) to set the DeckLink desktop to the same format as the video source, eg. NTSC 720 x 486. Next in the View menu of Final Cut Pro HD, set External Video to "Off". When finished with the Voice Over Tool, set External Video to "All Frames" to use the broadcast monitor."