It's been all but confirmed by Rockstar themselves now, the question many people have been wondering about has finally been answered. Vice City Stories does indeed look like it's coming to the PS2 after all.
Many people speculated that Rockstar would not bother porting the game to the PS2, seeing as we will see the release of Grand Theft Auto IV
this October dwarfing all other GTA games, but a search on the official ESRB website for "Vice City Stories" now lists a PlayStation 2 version of the game too, with the same 'M' rating and descriptors as the PSP version. This means Rockstar must have submitted the game to them.
See for yourselves on the ESRB Search page
for "Vice City Stories". We'll hopefully have a press release from Take Two soon, confirming that this is correct. Stay tuned for more VCS and GTA4 updates.UPDATE
: After reading some of the comments here I checked other countries rating sites and was unable to find any other mention of a PS2 version. The BBFC (UK) only has the trailers rated for the UK, PEGI (Europe) only has the PSP version, and while the OFLC (Australia) does have a PS2 version rated, it doesn't have a PSP version there, suggesting the listing is a slight mistake by the webmasters when it was submitted back in September. Thanks to Neil for the heads up.