Thank you Lee,
First, it is understandably upsetting, as seizures are never nice to witness. Now the key to how to approach this situation does completely depend on his current state and whether there is any chance he has had access to toxins (plant toxins included since you mentioned that he vomited up a twig and there is a possible seed pod exposure). If these are highly likely to be dog unfriendly plants, then an urgent check up would be best.
If they are not, then we can monitor and offer some supportive care to Barney. Now fits post-vomiting are not usually due to vasovagal responses (these tend to result in fainting). Instead, we can see vomiting lead to increased intracranial pressure. The increased pressure on the brain then triggers the seizure. So, it is quite possible that the force of vomiting lead to the fit (instead of their being an underlying cause for the seizure).
Therefore, since he is well in himself just now and as long as he is appearing mentally normal at the moment, we can initiate some supportive care to reduce his nausea and GI signs. To start, if it has been a wee bit since his last vomit (since otherwise he may need to have his stomach rested for a few hours first), then you can consider treating him with an antacid to settle his stomach. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the ones I tend to use are Pepcid (More Info/Dose) or Zantac (More Info/Dose). These are usually given 20 minutes before offering food (to allow absorption) and of course you want to double check with your vet if your wee one has a pre-existing condition or is on any medications you haven't mentioned.
Once that has had time to absorb and is more steady on his stomach, you can consider starting him on a light/easily digestible diet. If you do so, start with a small volume (a spoonful) to start. Examples would be cooked white rice with boiled chicken, boiled white fish, or scrambled eggs (made with water and not milk). There are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). When you offer that spoonful, give him 30 minutes to settle. If he keeps the food down, you can give a bit more and so on. As his tummy stabilizes, you can offer more. The aim of the easily digestible diet is that it will be better tolerated and absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As long as improvement is being seen, I usually advise that the diet be continued until the signs are settled, and that they are then slowly weaned back to their normal diet over a week.
Overall, fits are always stressful for any of us when the occur in our pets. Still the fact that he is currently well in himself, bright, and had no further signs means that we can monitor this situation and do not need to panic. Instead, we'd just want to address any nausea to reduce the risk of recurrence, monitor him closely, and as long as you don't think the twig was from something toxic (and he hasn't eaten those pods), then we'd assume the fit was related to the vomiting.
Finally, just to note in case you were keen to have him seen over this holiday, some veterinary practices in our country have office hours over the weekend. As well, I wanted to mention that most veterinary practices here do have contingency plans for emergency care for their patients even when they are not open. Therefore, it is worth ringing the practice. If they are open, you can get him seen today. If they aren't, then they will likely have a message to direct you on how to contact their out of hours service. And if you don't have a vet you can find one local to you, you can check the RCVS Register (HERE) to find your local vets or Vets Now (LINK) who are open all nights/weekends. In any case, if you wanted to get him checked out sooner then there are options to have him seen at any point this weekend too.
Please take care,