I'm glad to see more VC cosplayers coming out, I'm a huge fan of the game myself and would love to take on a VC cosplay soon.
Now, for Largo's lance. For the base I would use PVC pipe, and for three reasons: inexpensive, easy to find in that length, and most importantly it will be easy to set the lance up to "break" down into multiple parts as that's a huge prop. From the looks of it a good scale would be to make the lance as long as your brother is tall plus the length from your brother's mid-thigh to the top of his head. I wouldn't make it any taller than that depending on your brother's height and how much he can bench press as this will be a difficult prop to maneuver with. I build a 1:1 scale rifle for a friend cosplaying as Yoko Ritona and it ended up being too heavy for her and she's not a weakling. Long props displace the weight so while it may only weigh 50 pounds it will feel like more.
Back to construction... After you determine a good height for the prop and find a good size PVC (I'd probably use 1-1.5" white PVC) I would mark out your sections where parts of the lance will be. IE: First 2 feet will be the first lance/cone, next 2.5 feet would be the second cone and the flaps, etc. Use the measurements that correspond with what height you're using. Then mark where your lance will break away which will be screwed on and off using male and female connectors. I would recommend having the lance break away in areas where the connectors can be hidden by the parts that wrap around the lance or where the connectors can blend into the design.
If I were to make this I would probably work from the top down. For the very tip I would use the tip from those plastic lightsaber (http://www.getprice.com.au/images/up...aber.detai.jpg
) then build off of that. This is where you'll want to get some varying ideas because I'm not sure how good my theory is. Imagine the skeleton of a ship. In order to curve the bow you build the shape of your curve inside the body, then you attach the walls of the ship to that skeleton. For this you'll make rings all with the same diameter of the PVC but the outside distance is going to increase gradually until you have your desired shape. IE: First ring will be one-quarter inch from the outside edge of the inner ring, next one might be a half inch, then three-quarters of an inch, etc until you have a good shape. The placement of the rings and how many you'll make will depend on how long your lance is overall. I'd recommend using a 1/2" thickness wood that cuts and sands easily that's also readily available. Poplar would be my main recommendation. A drill press with an appropriate spade bit would be really useful but not exactly necessary. An electric sander, circular saw, or jig saw would all work tremendously well for shaping the rings.
Once you have the rings made and mounted I would probably use large sheets of styrene to shape the outside of the cones. Before attaching the outside of the cone you'll want to shape the rings. Use an electric to give the rings a downward slope towards the head of the lance. Be aware of how much thickness you have with the rings because you don't want to sand through them (Though with a 1/2" thickness that shouldn't be a problem). Use some clamps to mount the styrene, or whatever you're using, onto the lance and figure out where your cuts will be so the cone has a smooth transition all the way up. And depending on how large your sheets of styrene are, or how small, will determine how you'll want to wrap the cone (Length or width). Once you have the shape for the styrene cut out lay one edge along the rings and use a fast drying glue to mount the styrene. After that fully dries (I'd even wait an hour to ensure it fully cured) carefully wrap the styrene along the rings and apply glue to the rings while you do that. Do this for all the conic sections and you should have a pretty good looking lance.
Unfortunately for such a large project I do not know where to get sheets of styrene that large. But there are other options that would work. Non-corrugated cardboard (I'm not really a fan of cardboard but it is an option), poster board, craft foam, or anything suggested in the any of the "How to make armor" tutorials. I like the idea of using styrene, personally, as it's easy to use, thin, light, and bends wonderfully. But I don't know where you'll be able to find sheets that large as you'd need a lot. Perhaps someone else does.
Now I've never made this prop before, but if I were to this is what I would do. I'm sure there's an easier method out there that someone else could suggest, but your best option will be to try and get as many methods as possible together, then decide. You could even blend a few ideas together to make the lance even better.