You need to set the server IP. You can´t use master for all API access. It is server based. So if you direct the API request to the master, but the vps is on a slave you will get the vps id not found error.
I mentioned that many months ago, because this make tha Admin API unusable in my opinion., At least if used with automation software like WHMCS (there you only have the vps ID and WHMCS has no clue on which server the vps is installed.).
That´s one of the reason I never use this.