Can someone modify my listing?

You can only change prices related to the listing you have linked. 

If you have added other listings to Vayoo, you might have seen the option to change its prices, but you will not be able to do it. For that, you will need to previously link that listing to your Vayoo account.

In other words, even though you have seen the button "Change prices", you will not be able to change the price unless you link that property.