The reason it isn't the same as raising you idle is that you are introducing a fraction of the air compared to if you raised the whole slide. The key is to just add a little eddying air to lean the off-idle so you don't have any bog.
I have used this on non-vespa's alot. Typically I find this moot on Vespas as I would just clutch the bike into the pipe, thus avoiding the low bog.