As far as I have been able to determine, the Mono Mode with Transition Samples is really nothing more than an integration of the 'legato' technique from GOS' Maestro Tools. While that can be fairly effective at times, it does not allow (as far as I have been able to determine) for the incorporation of full interval samples in the manner of VSL - only one set of transition samples (which is a shame really..). Of course, the information for this feature is sparse (more like non-existant) so I could not say for certain if that is the true extent of its capacity.
It seems to me that what is needed is a combination of this with the Portamento Filter - defined start and target sustains, with the middle slur provided by the filter.
To use full intervals you have to use the other Legato Mono Mode feature - that works quite well. Just make sure that the dimensions are in the order specified by the documentation - if even one is off, then it will not work properly!
I have just determined that my problem was that the instrument was at 16 bits, and neded to be at 24 for the rule to work. Initial attempts at a 24 bit instrument reveal a lot of tweaking to be done.
I will keep experimenting.