It seems that in this country, vintage bikes get ripped off to get used as paddock bikes - well, I guess the same happens with most dirt bikes that get stolen.... But if you were a thief, and were presented with two potential targets, both with equal security measures, you'd almost certainly go for the new bike over the vintage one too...
I like to think that the VMX community is too small to steal a race bike.
Which is not to say that (despite living in a very low-risk area), I don't have all my bikes' numbers recorded, and am not careful about keeping them mine....