1983 should still be a metric seat tube (i.e, 28mm nominal outer diamter).
Vitus 181 nominal seat tube thickness at the post opening was 0.7mm
Therefore, nominal inner diameter is 26.6mm[ I.e. 28 - (2 x 0.7)]
Subtract an additonal 0.2mm for post to seat tube clearance and the theoretical seat post size is 26.4mm.