As well as the obvious meaning, a "servante" is a bench vice. "Etabli" seems to imply that it is fixed to the bench, in which case the "mobile" may mean that it pivots (as some do) about any one of a number of axes (usually vertical). On the other hand, it could be referring to an engineer's vice of the type used on pillar drills: these have slots for clamping bolts and can be moved around on the platen.

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Note added at 18 mins (2012-09-19 12:13:38 GMT)
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Example of tilting vice: http://www.axminster.co.uk/axminster-axminster-tilting-drill...
and drill press vice: http://www.toolstation.com/shop/p23292

See below what French manufacturer Facom sells under the name "servante",
http://www.facom.com/fr-fr/p-127-Servante-Jet-XXL.html
The Jet XXL could be closer to an "établi" as Chris Collister mentioned: rather a bench with the optional wooden top

The most common model among mechanics is a mobile tool chest with a lock in which every mechanics keeps his tools
http://www.facom.com/facom/Servantes/index.html

So the standard model of "servante" is a tool chest on wheels, with the height of a bench, and some models may receive a wooden top, the vice being in my opinion rather reserved to the "non-mobile" models larger and fixed (Jet XXL above, less common because much more expensive than regular benches or tool cabinets)

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Note added at 1 heure (2012-09-19 13:32:10 GMT)
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Most basic item with no options:
http://www.caisse-outils.com/servante-atelier-vide/22-servan...
Full range of "servantes" (catégory "vide, the other one is full of tools):
http://www.caisse-outils.com/servante-atelier-vide/22-servan...