Watching this US internecine warfare from afar is fascinating and somewhat amusing.
There are seven current signatories to the agreement, China, France, Germany/(EU), Russia, UK and USA on one side and Iran on the other. A veto-proof "no" vote in the US Congress will remove the US as a party to the agreement but, believe it or not, have no effect on the other parties who are showing no signs of backing away. China, in particular, has the "New Silk Roads" project in which Iran has an important place. US machinations in Ukraine and the EU have pushed Russia back into a friendly relationship with China and the EU countries, France, Germany and the UK, are in no economic position to refuse a new trading partner. The recent reopening of UK and Iranian embassies in each other's capitals is a sign of the times. US and Israeli warmongers wishing to launch an unjustified war on Iran are unlikely to be able to raise a new "coalition of the willing" and could face active resistance from China and Russia.