this can happen if you open a "corrupted" model. These are not really corrupted because you can still use them ingame but VM cannot open them.( VM has also many other bugs that never got fixed :/)
And I don´t have an idea why you cant see vertices sometimes. Could you search for a model please with that bug and pastebin it to me so I can take a look on it??
A solution could be to search for another model with a similar shape and then using this1 instead.
Or you could try to convert the model to mdx and then back to mdl.