We may not have Cirsium arvense but Australia has plenty of thistles including Cirsium vulgare (spear thistle) although most of ours aren't Cirsiums and I suspect Vietnam would have its fair share too. Not too sure why the Viets signalled arvense out but it maybe a nasty one or just another excuse to leverage suppliers.
If they think Australia is going to increase their supplies to Vietnam this year they may be in for a surprise, our eastern states crop is way down this year (and being battered by some weather right now) and will all be vacuumed up by the domestic market at quite a premium to the international price and although WA has a good crop a fair bit of it will go east to collect the same premium. Australia's exports this year will be well below average so the Viets may go a bit hungry if they ditch their other main suppliers because of some thistle seeds.
