Hello, my dog is a 16yr old Shih Tzu, I took him to TCAP (a low cost vaccine clinic) to get his Bordatella, Leptospirosis and DAPPv vaccines. Before the vaccine he seemed a little sluggish but I just thought he was stressed. He still had muscle control and stuff. After the vaccine I handed him to my mom int the car and she ran back into the clinic 2 minutes later with him limp in her arms. His tongue and gums were white, he was making this horrible snoring noise, and he was completely unresponsive. His heart rate was a bit slow aswell. We rushed him to a real vet and she said his chances weren’t good and he probably had a stroke. They told us to put him down but we decided to try to save him. They gave him steroids and fluids, not sure what else they did but by the end of the night he was walking around and eating. We brought him home the next day, so far he’s been a little groggy but hes not dead which is good. He’s on tramadol and steroids, today we stuck to steroids and he seems a little more coherent. Now I’m curious though, what the hell happened to him and should I keep my expectations low? The vet offered a stroke or a embolism, but I’m curious if it could’ve just been vaccine reactions even though it happened so fast. I’m very worried about him and we’re taking it easy. He’s on Hills urgent care diet with boiled chicken for now. If you have any ideas as to what could’ve happened please let me know. 
First pic is before the vaccine second one is after the emergency vet 

Help me figure out what happened to my Shih Tzu (the vet doesn’t know)