It seems the algorithm only allows you to enter a single value, not a value that changes. I think this is also the case in the updated version of the tool for QGIS 3.0.
However, you could edit the source code for the algorithm slightly and have it read the values from a specific field. In my case, I am using QGIS 2.18.13 for Win7 64-bit. The file for the algorithm can be found in:
C:/Program Files/QGIS 2.18/apps/qgis/python/plugins/processing/algs/qgis/DensifyGeometries.py
processAlgorithm() function, we can see where the vertices are called:
We can replace
4 is the field index containing the values (you can check yours by right-clicking your layer and going to
Properties > Fields and check the number in the
Note: Depending on permissions, you may have to move the .py file outside the folder such as to the Desktop, edit and save the file, then insert it back to the same folder.
Restart QGIS then run the tool again on your layer. You can ignore the default vertice value as the script no longer uses this to densify the geometries.