A network generated completely independently of VERTEX (e.g. MediumNet here: https://github.com/OpenSourceBrain/ACnet2/tree/master/neuroConstruct/generatedNeuroML2) could save spike times/voltage traces.
These files (NML2 files & spike times) are read in with a MATLAB script & the internal structures in VERTEX are populated and the methods for generating/analysing LFPs are used on the data