Because it's a wet hull, the amount of lead needed depends on the size of cylinder used, so it will vary from Vanguard to Vanguard, even though they're the same size boat.

The easiest way to get a pretty accurate estimate is to first calculate the volume of your cylinder- 3.14x cylinder radius squared x cylinder length.

Your cylinder is 10cm diameter, not sure on length, but it looks about 90 cm so I'll use that figure as an example.

3.14x25x90= 7065 ml displacement or 7.1 litres. 1 litre equals 1kg (metric makes things so simple) so that's 7.1kg.

Weigh the cylinder with all it's components installed, lets say it weighs 2kg with everything plumbed in, then subtract that from the 7.1kg. Subtract a further 0.5kg to allow for the extra density of the GRP hull, and you will also need to factor in the lead acid battery which will weigh considerably more than the water it displaces. I don't know what figure will be, but it's easy to calculate- area of base times height, all in cm will give you the batteries displacement in ml. divide that figure by a 1000 to give you a figure in litres.

Weigh the battery in kg's, and subtract the displacement figure. Lets say the figure you get is 0.5kg.

So for our hypothetical cylinder, we have 7.1-2-0.5-0.5= 4.1kg of lead

This is gross simplification, because there are many things I haven't factored in, like the heavy metal propulsor at the stern, the external tie rods, and even the displacement of the lead weight itself, but it gives you an idea of what you need, and you should get a figure between 10-15% of what you need.

Regarding foam, most people use the closed cell pink or blue coloured foam, it's quite firm to the touch. But there it's quite likely you won't need any.

All the above assumes the cylinder sits completely below the waterline when the boat is surfaced. Anything that sits above will reduce the amount of lead you need in the keel.