What does VIB stand for in EICAS?
These are engine vibration measurements. The image you show is the Compact Engine Display:




  3 Vibration (VIB) Indications
  
  Displayed as digital readout only.


(Boeing 737 NG FCOMv2 - 7.20.1 Engines, APU - Over/Under - Displays)

The value comes either from the EEC (Electronic Engine Control) or the sensor directly:


  Oil pressure, oil temperature, oil quantity, and engine vibration are the secondary
  engine indications. [...] Engine vibration indications are displayed with a round dial/moving pointer.
  
  The EEC must receive electrical power to supply engine operating data to the
  flight deck engine indications. When the EEC is not powered, N1, N2, oil quantity
  and engine vibration are displayed directly from the engine sensors. Positioning
  the engine start switch to GRD supplies electrical power to the EEC and displays
  pointers/digits for all engine parameters.


(Boeing 737 NG FCOMv2 - 7.20.1 Engines, APU - System Description)

On the lower DU they look like this:



The amber warning line (2) shows the limit of 4.0 units. The QRH defines High Engine Vibration as:


  
  The vibration level is more than 4.0 units
  Airframe vibrations.
  


(Boeing 737 NG QRH 7.24)



As to what the actual number really means, the FCOM does not say. I could find this quote on pprune.org, but I cannot verify it:


  Two types of vibration pickups have been used on Boeing airplanes, the electomechanical velocity pickup and the piezoelectric accelerometer. Signals from velocity pickups are converted into vibration displacement, measured in 1/1000 of an inch, peak to peak (mils double amplitude). These units represent the physical displacement of the vibration sensor. Signals from accelerometers are converted into velocity, measured in inches per second (ips). These units represent the peak amplitude of the velovity of the vibration sensor. Boeing airplanes typically use a 0-5 scale for cockpit display of vibration. On earlier applications, the scale provides an indication of absolute vibration units (ips or mils da) as output from the airborne vibration monitoring (AVM) signal conditioner. This approach is not optimum in that a display of displacement units will provide relative insensitivity to low speed rotor vibration. AVM signal conditioners used on 737-300 and latter, 747, 757 and 767 provide output data in scalar units. These scalar units are uniquely tailored for each engine type to match the operating speeds and vibration characteristics of the engine rotors. The scaling used on the 737/CFM56 installation provides display sensitivity for the high speed rotor proportional to the vibration velocity. The low speed rotor vibration is displayed in units proportional to displacement, except near idle conditions where velocity scaling is used.


(Boeing Airliner magazine, OCT-DEC 1987 issue, emphasis mine)

So the number was originally based on acceleration in inches per second, but it is a scalar value today without a direct physical meaning. The only thing the pilots need to know is: around 1.0 is normal, 4.0 is the limit.

This page confirms, that scalar units are used on the CFM56-7B, which is used on the 737 NG series:


  The algorithm detects the highest vibration (in scalar units) of all four sections of the engine during the whole flight and takes a snapshot of the remaining parameters. The vibration data are transfered to the next flight leg upon engine start.
  
  The Vibro Meter Advanced AVM Signal Conditioner (AAVM) P/N 241-298-002-011/-015 is able to detect #3 or #4 bearing vibrations based on the CFM56-7B engine vibration signature.


(AviaDeCo GmbH)