Hi Emo,
could you e.g. post a picture of it?
btw. are there any edges connected to the vertice?
I usually can't delete vertices if there are 3 or more edges connected to it, if there are more than 3 edges connected to the vertice I have to delete these edges first... an other problem I sometimes have is if the vertice is on a borderedge (Display --> Polygon components --> border edges)....
sometimes I can't delete the vertices with the delete button, but I have to use the delete vertex function... (Edit polygons --> delete vertex), but it delete both the vertice and all the connected edges... so I usually use the other method since I usually want one of the edges to stay...
I hope this isn't to messy... 
regards
Strarup
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Alex V. U. Strarup
One mistake can ruin it all...