Why did the Vikings fail to conquer the Americas while the Spanish suceeded?Chapter Hemispheres Colliding Also,

The answer to this question is given quite nicely and
briefly at the end of p. 372 and on to p. 373 in my copy of the
book.

What Diamond says is that Norway was in the wrong
place and at the wrong time.  Norway was in the wrong place for two reasons.  First, it
was too far away from North America.  Second, it was too far north and so the islands in
between it and North America (Iceland and Greenland) were not good stopping points. 
When the Norse tried to colonize Greenland, for example, they found that its climate was
no good for agriculture.  Since agriculture was one of the main advantages of the
Europeans, this was bad.

In addition, the Norse came over
too early.  At the time that they tried to settle, technology was not yet advanced
enough to really help them.  For example, their ships were not good enough and their
weapons were not advanced enough.

So the Norse were trying
to colonize the wrong place at the wrong time.