Aaah, but the Pictish Body Art link (see above) which gave the Latin quotation, also suggests alternative translations and interpretations: - 'vitro' may not be referring to woad at all !
First meaning of 'vitro -i' is 'glass'. Alternative suggestions from the Body Art webpage are that the Latin is referring to 'glaze', or else that the 'inficiunt' may be a mis-transcription for 'infect', then giving the meaning that they 'infect themselves with glass' - which would leave a permanent raised pattern of scar tissue on the skin, as is also practised in other parts of the world, such as Africa.