How to "do, if expression increased"?

How can I code following pattern in C++ in order to execute statements inside if:

if ("vVector.size() did not increase")
//do whatever is here

?

NOTE: vVector is incremented automatically from time to time. It's actually a clone of another vector that collects things and expands without me having and willing to have control on it.

When I do:

if (!(vVector.size()++))
//do whatever is here

I get following error: lvalue required as increment operand