I was pretty amazed that nobody seems to have made a vector version of this picture.
Since I'm pretty sure I will need this in the next version of my mosaic generation, I eventually decide to make one myself.
Thanks to the help by CrunchNugget, it didn't take me too many days to finish this.
Of course it still took longer than it should, mostly because I basically never used Inkscape before.
I have tried to make this as close to the original picture as possible, but some minor differences still exist.
The most significant difference is the part next to Applejack's mouth. In the original picture there is a patch of white, but a careful examination shows that the white patch doesn't make any sense at all (it's not part of Celestia's body anyway), so I think it's better to get rid of that patch of white and replace it with more reasonable background.
According to my earlier plan, the next version of my mosaic won't be generated directly from this picture.
Most likely I will change the background to balance the color distribution. Some other minor adjustment is also needed to make it more suitable for mosaic generating.
Mane6 and Spike: Based on the SVG by CrunchNugget [link]
Celestia: Based on the SVG extracted (from official flash file) and corrected by CrunchNugget
Logo: Based on the PSD by metalbeersolid [link]
and converted to SVG by me
Everything else: me
MLP:FIM is (c) Hasbro, Studio B, and Lauren Faust.