Go to your local car salvage yard and get a used tank and replace it. If tank goes below the cold mark, you run the risk of pulling air into the cooling system. You could then run the risk of over heater the engine as air can get trapped in the cooling jacket and cause hot spots and steam to form. The tank is there to keep the cooling system full of liquid at all times (no air). When the coolant expands and contracts as the engine heats and cools, this tank is where the coolant is pushed and pulled from by the radiator.
It should be easy to replace. I have the second generation Vibe and the process for the tank removal is as follows:
1. Remove the cap from the radiator surge tank.
2. Remove the 3 bolts (1, 2, 3) and radiator surge tank assembly.
I would think the first generation Vibe would be about the same. Three of four bolts and a hose at the most to remove. It should be very simple if you look at it. If you can get a used tank, you would be much better off an trying to glue it. The heat from the coolant and the vibration from the car does not work well with trying to glue the tank. The oils in the plastic of the tank is not going to make for a very good bond of the glue to the tank. If you want to repair the tank then have it welded. The weld has to done with the same type of plastic as the tank is made of and there are a lot of different types of plastic. A body shop that welds bumper covers or a shop that makes plastic tanks should be able to weld this if they have the proper welding rod. It would still be faster and I would think cheaper to get a used tank from a junked Vibe.
If you understand how the Radiator Surge Tank works (think of a straw in a soda bottle), there is nothing magical going on. You could replace the tank with any container or bottle. The trick is finding a container that will fit in the space and be at close to the same height as the original tank.
The reservoir and radiator fan and housing are all one thing, so if you replace it, try one from a junkyard, a new one would be a lot more pricey, if you can find one.
There are things that go wrong with cars that need immediate attention, and others that are not immediate. I'd put this as something you should deal with sooner than later, but its not the kind of thing you should park the car for until its fixed. As others have said, its not good to lose coolant, but a leaking radiator would be worse than a leaking reservoir.
The commercials on tv say that flex seal stuff is a miracle. Obviously, nothing advertised on TV is as good as the commercials say it is, but something like that might work as a patch, don't know how well it would hold up to higher temps, but the reservoir is not pressurized. You'd want to clean the surfaces well before attempting a surface patch, and if a replacement part is expensive, you could try a cheap patch and see how it holds up
Assuming that the crack has caused you to lose some coolant, keep in mind that when topping off the coolant, you should use the same kind as what's in there. The cars came with Toyota red coolant, but its hard to say what might be in a 13 year old car. If you've only lost a little, you could top it off with some distilled water.
I have checked the ebay for you. The fan frame seems to be sold with the fan together. The used ones cost from 50 to 150 dollars shipped.
But notice there is a lower hose which goes into the radiator. The reservoir can be replaced by any durable container that connects to this hose. Then tie the container somewhere using zip ties.