Parts list for vivitar tripod model 1100

So is this knob positioned on the lower, centre tube between the legs? Could you post photos of the Knob and where it fits to the tripod?
Most tripods have adjustment for 4 settings. Height, rotation, Tilt and mounting plate angle. We'll need to work out which one of these it's for. Height is usually a crank handle so it's possibly not that one. The other 3 are usually a bolt with a moulded cap to make turning it easier. Some tripods have a screw clamp on the body to lock the legs into the open or closed positions.
Comment below with more information and I'll try to help you more.

Two potential issues. 1. Perhaps you have just not screwed it on tight enough. Just tighten the black knob as much as you can then screw the head on your legs as tight as you can. 2. If you can't even do that I will assume that the hole on the bottom of the head is too large for the screw on the tripod legs, in which case you just need a a bushing adapter. Like this one:
http://www.bhphotovideo.com/c/product/882988-REG/Sunwayfoto_tn_15_Bushing_3_8_1_4_Stainless_Steel.html

That means the other two legs are connected with the very hardware you're looking for. Why not remove them and take a trip to the hardware store for a match?

If the head is loose on the tripod follow these instructions:
There are three small set screw (grub screws) that are under the head, net to the center column. You will have to raise the center column and look under the head. These are slotted screw heads and a small to medium screwdriver should work. You don ot have to remove the grub screws completely, but you should back them out at least half way. The head will unscrew off the center column. The gray part below the head is half the head attachment and half the center column end plate so it should separate in the middle line between the two.

If screws came out elsewhere, we will need to know the model number of the head that came apart.

There are three small set screw (grub screws) that are under the head, net to the center column. You will have to raise the center column and look under the head. These are slotted screw heads and a small to medium screwdriver should work. You don ot have to remove the grub screws completely, but you should back them out at least half way. The head will unscrew off the center column. The gray part below the head is half the head attachment and half the center column end plate so it should separate in the middle line between the two.

Try www.TripodQuickRelease.com for that part. There is listed the widest selection of quick releases and support more brands than any other web site I’ve seen.

Because of the age of the tripod, and the fact that Vivitar no longer sells or supports tripods, I think you would be better served if you bought a new tripod. Slik and Manfrotto are two that will offer you a great price, durability and in the case of the Manfrotto, parts available from a wide range of sources including the USA distributor: www.bogenimaging.us you can see tripod and parts at www.BogenTripodParts.com Otherwise you could try ebay or craigs list to find the same model and use it for parts.

There are three small set screw (grub screws) that are under the head, net to the center column. You will have to raise the center column and look under the head. These are slotted screw heads and a small to medium screwdriver should work. You don ot have to remove the grub screws completely, but you should back them out at least half way. The head will unscrew off the center column. The gray part below the head is half the head attachment and half the center column end plate so it should separate in the middle line between the two.

'camera mounting scrwe'..any good camera store or online... samy's camera for example

Carole31,
I have the same tripod, and the screw shown in the photo you posted is to lock down the rotation of the tripod's head, i.e., what a video camera uses to pan (horizontally) across a scene. The large knob is used for the same in the vertical direction (up and down motion).