Check again, it vibrates two times, then it just continues without vibration. This is such a fail i have no words.. and it's not mentioned anywhere to be fixed for 8.1.
Ok, I understand. I didn't even ever payed attention to that even after three lumia's...simply because it is kind of trivial.
If it were the other way around, and the sound would cut while vibrating, that would have been really annoying, which is what I first understood what was happening to your ringtones...
Preferences vary, so for some it may understandibly be a huge bummer getting only two vibes, but for me personally vibe is supposed to be felt, and imo if you don't feel it the first two times, you won't feel it anyway.
The sound is to be heard, and as long as that don't cut, if you haven't felt it, you'll hear it. Also, more vibes would eat more battery, so maybe that's how you could try to see a positive side in it.
Don't actually know why they made it that way...