Attempt to bring the two nations of the Scots and the Picts together. Two strong kingdoms are better defense against the might of the Vikings.
They are reasonable people. When they attack, welcome them with open arms. They may agree to sit down and strike a peaceful solution – instead of your head!
Play a card
Although in the past the Scots and Picts have had a, shall we say, 'conflicting' relationship, you need to reconsider your needs.
The Picts can be fierce warriors. With the Vikings putting pressure on one side, you need all the support you can get from the other.
It would be very unwise to surround yourself with enemies. Wouldn’t you rather the Picts were friends rather than foes?