Tamron 28-300 VC

I purchased the above lens a couple months ago as a walk-around lens to eliminate having to lug a lot lens on my bike. I was fairly satisfied until about three weeks ago when the zoom mechanism broke. Back to Tamron for repair since I had cut out the code on the box to get rebate. Just got the lens back and am ready to start screaming at the dealer and/or Tamron for a refund. First though, I thought I would run this by you guys.

My problem is shooting at the 300mm end. Today, I shot the following image at 300mm, f/6.3 and 1/500 sec. I expected the image to be sharp since the VC should give me at least one more stop and eliminate any chance of lens movement. As you can see in the below crop, there is slight blur caused by lens movement.

Am I expecting too much? My hands are definitely not as steady as they once were and I like the lens but this aggravates me. Here's the full shot and a 100% crop.

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