Currently, the limits on accessing the VRAM of the various TMS VDPs are not emulated.
The VRAM access timing is very accurately described in section 2.1.5 of the application manual:
http://map.grauw.nl/resources/video/texasinstruments_tms9918.pdf
Also there are some interesting tidbits of the inner workings of the TMS VDP range in these records of an interview with one of its designers Karl Guttag: http://spatula-city.org/~im14u2c/vdp-99xx/
Mostly implemented in commit ddcd3a47a22375a1266a405bf23c6c7b8c0de457 by Wouter (thanks!). But it is not exactly known/sure yet what the value for the delta is on the TMS, see VDP.cc:
If I understand correctly, Wouter wants to keep this ticket open until the definitive value is determined (how?).