Here is a simple Plain TeX snippet for demonstration
\vskip 3in Aaaaa \vskip 3in Bbbbb \end
When I compile this with pdftex, I expect to see a 3-inch-high blank space, then Aaaaa, followed by another 3-inch-high blank space, then Bbbbb. But the first \vskip appears to be ignored by TeX.
On the other hand, when I do this
\hrule height 0pt \vskip 3in Aaaaa \vskip 3in Bbbbb \end
The output is what I expect. But I see no reason why a dummy 0-point-hrule is necessary.