does a portable mp3 player battery charge last less playing vorbis than playing mp3s?

It did on my LG portable I had once. I did not measure it but I estimate that ogg playback was like 60 per cent of the battery life compared to MP3. It was LG so I blame crappy ogg implementation in the first place.
I guess it makes more difference on the cheapest devices (small screen, sigmatel single-chip design) than on those featuring a "normal" CPU (usually MP4 portable players capable of video playback).

Unfortunatelly vorbis decoding is more computatively intensive than mp3, so yes, it makes sense that it should consume more power, specially in embedded systems such as DAPs.

I concur. This is something I searched through forums for a few years back and found that it is in fact the case. I have an iAudio X5 where OGG files eat up a lot more battery power than MP3. I suppose the smaller file size of OGG for comparable aural quality against MP3 is the advantage.