How can I make vfill to work without adding any text before?

Adding an empty box like \mbox{} or \hbox{} (or \null for short) before the \vfill makes it work.

\newpage
\null
\vfill
blabla

LaTeX provides the macro \vspace for adding an amount of space, \vspace{\fill} has the same effect as \vfill, and has the same failing. LaTeX also provides¹ the macro \vspace* for "I really, really do want you to put the space in here." so \vspace*{\fill} works. That is to say,

\newpage
\vspace*{\fill}%
blabla

puts the blabla at the bottom.

¹ Actually, \vspace and \vspace* are the same macro, but it's a convenient lie to think of them as variant macros.

It's important to understand how TeX's page break mechanism works, at least for the main aspects.

When TeX decides where to break a page, it pushes back to the "list of recent contributions" what doesn't fit in the page and does the break.

Usually what doesn't fit are the final lines of a paragraph, but a break can very well happen at a vertical space (such breaks happen always before the vertical space). Now the break is performed and the vertical space is put back to be reconsidered; but nobody wants to start a normal page with a vertical space, so TeX makes it disappear.

This unless this vertical space is declared with \vspace*: this kind of vertical space is never considered for a break, it's just like a big invisibile character.

How the page break was chosen is irrelevant: this happens even at the start of a document. If you say

\begin{document}
\vspace{3cm}

this vertical space will not appear.

Using \null after \newpage effectively avoids the disappearance of vertical spaces, because \null puts on the page an empty box before the \vspace (something like a zero width and height character).

Thus the "good" way to build a dedication page, for instance, is

\cleardoublepage
\thispagestyle{empty}
\vspace*{\stretch{1}}
\begin{flushleft}
\itshape Dedicated to the people on \TeX.SX\\
         on the occasion of the first birthday\\
         of that wonderful site
\end{flushleft}
\vspace{\stretch{2}}
\cleardoublepage

The \vspace*{\stretch{1}} is actually equivalent to \null\vfill; but specifying the stretching with \stretch allows for nice effects: here the white space after the dedication is twice as much as the space before it.