I originally purchased a swivel-base vice to go with my mill but the swivelling part was ditched in pretty short order, never to be used. I sold it after a while and replaced it with a 100mm verstaile SG vice from my favourite Leicestershire emporium. Actually I ordered a 160mm one, only to find out it was far too big and heavy for my mill and nearly gave me a hernia trying to lift it onto the table. Luckily, I was able to return it straight away and trade down to the smaller one. You might have trouble mounting it longitudinally, if you needed to. Personally, I've never found the need. I do, however, find that the alternative vice jaw positions can provide useful solutions to intricate workholding problems on odd occasions.
John
( Warco VMC mill, not a WM16 user. )
Edited to correct spelling
Edited By John Hinkley on 20/06/2019 10:33:30