Re wearing a weighted backpack in lieu of a weight vest:
IMHO this can create an unnecessary risk of back strain, because the weightload is concentrated in one singular area. I recall reading that children and indeed adults have that problem with overloaded backpacks that they carry around for their everyday use. Plus, because you're overcompensating for the weightload being concentrated in a smaller area, your form for the squats, lunges and any other weighted leg movement can be compromised, diminishing the effectiveness of the leg work.
The AllPro and most other weight vest brands are such that you can spread the weightload (the AllPro model has a bunch of little 1/2-lb rods that you slide into pockets sewn into the vest) around the entire trunk area (front, back and sides) as well as up the middle of the back. Thus the weightload is far more evenly distributed.
JMHO -
A-Jock