It feels like I never have the right vise on the mill....
I own four different vises for my mill, three of them are three (3) inch models and the fourth vise is a 5 inch model that is frankly too big for my mill. I can't tell you how many times I feel like I have the wrong vise mounted for the job that I am working. At some point I need to order a four inch vise.
The three model covers about 70% of my projects. A four model would cover 95% of my projects. The rest of the stuff is so one-off that it is just odd and you need options.
If (when) I have a full sized mill the a four inch vise will probably the stand vise with a 5 inch unit for the odd jobs. --Dreaming of perfect machine shop future.