Soft Flasks with Normal Pack?

I rarely use hard bottles anymore, except for the 1L. Smartwater bottle that weighs next to nothing and tucks into the outside pocket of my pack.  I typically carry that plus the 0.6L BeFree filter with attached soft bottle, a 2L Platypus bag that holds dirty water for my gravity filter system, and a 3L Hydrapak Seeker soft bottle to hold filtered water.  (I like to set up camp and filter enough water for a day or so.)

To clarify, you are asking about how to carry a soft bottle that is accessible to drink from while you are walking, yes?  Unfortunately, I haven’t found a way to securely attach a Hydrapak-type soft bottle to a shoulder strap on a pack, and the side pockets on the pack I use most don’t really hold a full BeFree bottle very well because that bottle is so squishy.  The running vests you speak of usually feature an adjustable tensioner to hold the soft flasks in; my packs don’t have that, though you maybe could rig something up.  Not worth the effort for me.  When I have to carry water, I just fill the hard Smartwater bottle and maybe fill the Seeker, which is then stored inside the pack.

It is so nice to have these choices rather than the heavy Nalgene bottles!

Soft flask can be carried well in some backpack side pockets, such as the MLD Burn. On the Burn, there’s a shock cord draw-string at the top of the side pockets, so you can compress the side pockets well to hug a 1-L soft flask quite well. This makes it had to remove and replace the flask while wearing the backpack, but holds it snugly without lots of wiggle room.

But Jenny is right – one benefit of the BeFree if you have frequent water sources is you don’t need to carry any water.