Joined: Nov 12, 2011
Since the Vibe 300 uses a PP5020 SoC, this makes a big portion of the porting effort rather easy. The fact that it also uses mi4 files means that the porting effort may be even easier. You should read this page for some more information about the mi4 file format, and some useful pointers for it.
Take apart your device and see if there are any chips in it that may be in use in existing targets. I know next to nothing about the Packard Bell DAPs, but I'm banking that the Vibe 300 and Vibe 500 may use similar hardware, apart from the LCDs.
On one last note, the pictures of the Vibe 300 that I'm seeing on Google show that it has a portrait oriented screen. Since you mentioned it having a 160x120 (possibly 160x128) resolution, then rotate that to portrait orientation and you have a 128x160 LCD.