Depending on how both do in the spot vigor of clematis will probably be a little greater than that of the grape. Usually when you put two climbers together you have to do a little editing in order to maintain harmony anyway.

There are other clematis planted here that grow much larger than this one. Maybe their idea of a thug is anything more vigorous than 'Jackmanii'.

I'm going to rip out one of mine. I'm very happy with 'Helios' but 'Bill MacKenzie' is growing really wide and in every direction but up the tree where I want it to go. It must have thirty vines going off in thirty directions and when I try to direct them to where I want them they break off.

I'm not sure it was properly labeled since it is blooming sooner and growing shorter than it is supposed to but it is a tangutica for sure.

If anyone in the area wants it, come and get it....

I've got one and it's a rampant grower, needs vigorous pruning. I made the mistake of not cutting mine back for two years. The third spring I cut it back to the ground, a whole truckload to the landfill. It's up and growing like gangbusters once again, I will be cutting it back every spring from now on.

I have 3 tangutica varieties, and they have all been well behaved. I have them growing near large trees so they have a lot of root competition. The poor growing conditions may keep the size down. I have never had any seedlings either.

Mine grew very large--too large for my taste.