Hi Jin Hwee,

Thanks for you recommendation! We're glad that our product fits you running vest of choice. As our water Bladders are made to fit in our Decathlon running vests, we can't say for sure that they might fit into the vests of other brands so it's a good idea to bring you vest down to see it they actually fit. 
Yes, 1 litre is a good volume for a mid range distance run. If however you find that you need a little more due to hot weather, you can consider getting out Soft Flasks that come in 150ml, 250ml and 500ml volumes. 

Your Friendly Neighbourhood Jogger,
Randi