Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
Poor Flash, the vomiting and attempts to eat the plant suggests nausea (eg stress, gut infection, pancreatitis, dietary indiscretion). Since there was mouse poison there, we need to be careful here but cats usually don't eat this (its more a problem we have with dogs). In any case, since most mouse poison are anticoagulants that take weeks to show signs -- plan to keep a close eye on his gum color (that it stays pink, not white) and any signs of bleeding or breathing changes. If we saw those, we'd want him seen. One other thing though less likely, keep a close eye that Flash can pass urine. It should be fine but sometimes we see male cats get bladder blockage when stressed and they can present as vomiting initially (From toxin build up in the blood).
Those aside, for the vomiting, if it has been the once, we can monitor over today. You could also counter that nausea with an OTC pet safe antacid [ie Omeprazole (0.25-0.5 mg per pound once daily), Esomeprazole (0.25-0.75 mg per pound once daily)]. Whichever you choose, give it at least 20 minutes before offering food to allow absorption. Though if he cannot keep this down, then that's a red flag that we need a local vet to bypass his mouth with injectable anti-vomiting medication.
Afterwards, let's start small meals of a light diet (eg boiled chicken, boiled white fish, or meat baby food (as long as its garlic/onion free)). These are easier to digest when the gut is not feeling 100%.
Overall, his signs do raise concerns but not likely from the mouse poison. Still keep a close eye and let's start supportive care now. If he cannot keep that or water down at any point, appears weak or dehydrated, or doesn’t respond to the above over 12-24 hours; then we'd want to get a local vet to assess him for the above. Based on their findings, they can treat with injectable anti-vomiting medication +/- antibiotics to get him settled.
Kind regards, *****