Any chance you could have it delivered to a different address??
I had the same problem with my mum,though it wasn't a vibrator,it was a motorbike jacket and she would of been furious if she'd known that my BF had a motorbike (death trap etc).I had a really good friend who let me have it delivered to her house.
If thats possible you could then just pop the vibe in ya bag and you are spared the blushes!
Plus i'll give it some thought today to see if i can think of some excuses! LOL ;)