make it official la, adopt him.
whiskas, nope

Whiskas as treats isn't bad, since it's not going to be the main source of nutrition. But I would not recommend the brand as food.

just bones? 

I'm not an expert on this but try change food first. If your cat still vomiting after changing the food, bring your cat to vet. If there is blood in vomit, bring your cat to vet asap.

You can read this article, might be useful.
http://pets.webmd.com/cats/what-causes-cats-vomit

Hmm what food do you usually give your feline? i.e. before you switched to Whiskas Mackerel ?
I think it could be due to a sudden change in the cat's diet. Any change in the cat's diet would need to be done gradually.

perhaps you may want to fed your feline the usual diet, and mix it with the new diet.
Just my two cents.

^ LOL if that is the case, then okay la

I'd like to bring back to my earlier questions :

Alright, with the cost of living and the GST looming around the corner,
I am thinking to mix my cat diet between BW, and a lower range cat food.

Can anyone suggest which is the best options amongst these brands : Whiskas, Friskies, SmartHeart, proDiet.

For food? Brands are not important. It is the ingredients that are. I personally avoid corn, wheat, gluten. I've found that grains are not bad at all - my two girls are flourishing. But really, pick a brand that you think is good based on your research and knowledge, and if it makes your cat's coat shiny smooth and sleek, with healthy bowel movements and no vomiting, then stick with that.
Read above.
Unfortunately, all brands are commercial. All pet food manufacturers are in it for the profits, so it's really up to us to decide, based on ingredients (which can't even be 100% trusted), what is best for our pets.

PEOPLE NEED TO STOP THEIR ATTACHMENT TO BRANDS AND READ THE LABELS INSTEAD. CHOOSE BASED ON INGREDIENTS, NOT NAME.
What works for one cat, may cause allergic reactions in another. If your food works for your cat, and someone else tells you otherwise, ignore them. Your cats are healthy. Don't mess with that.

What Exactly Are Animal By-Products?
Basically, animal by-products are what’s left of a slaughtered animal after the edible parts have been removed. They include the waste2 of meat processing not intended for human consumption. For example…
Feet
Backs
Livers
Lungs
Heads
Brains
Spleen
Frames
Kidneys
Stomachs
Intestines
Undeveloped eggs

That not necessarily means they're bad though. Take an example of chicken by-product; chicken feet is mostly bones. It contain important nutrients, calcium and phosphorus. Chicken eggs and organs all contain great nutrients.

EDIT: BTW, I just googled about Intestines. In order for it to be included in Chicken by-product, they need to be cleaned properly. That's the procedure. I also read avoid buying food that contain anonymous by-product because we don't know the source.

Can i give fish oil to cat?