Maybe the skill of the person who repairs it matters, transfiguration master quays get it perfect but for normal people it deteriorates

 

If you can't completely, just add a little conjuration and voila. Good to go.

 

I always thought Reparo worked based off of the mind's interpretation of how something should be repaired. (Which is why Dumbledore could repair on a much larger and complex scale than others - mental control, perhaps?)

So if you had conjured/transfigured pieces lying around and you believed that the charm would work with those pieces and the vase then it probably would work.

Another fact is that there's been virtually no advancement in materials science in the magical world for the past... 200 or so years in canon? Castle is all stone. Windows? Brooms and wands are wood. Virtually no modern materials are used in all of canon. This implies that there was no need for more durable materials, in other words, the repair charm and other charms worked fine, and continue to work fine.