it says you must add and manage your contacts on a computer or other skype compatible device and changes (contacts) will automatically sync when you sign into skype on the Vita.
It's under the important notices on the Skype app download in PSN (Sen) store.
Here are some useful tips on how to use Skype for Sony Vita: https://support.skype.com/en/faq/FA12017
As in regards to adding contacts- unfortunately it is not possible. You need to use a different device for that purpose.
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