Yes, the native PHP [highlight_string](http://php.net/highlight_string)(), for example, works as expected. vBulletin, however, seems to replace one or more backslashes with none.
However, you can use the other syntax highlighter (which I presume is GeSHi)
<?php
use Foo\\Bar\\Baz as Quux;
echo Quux\\somefunc();
Actual BBCode:
[noparse]
```php
<?php
use Foo\\Bar\\Baz as Quux;
echo Quux\\somefunc();