Parshat ויקהל is mainly a summary of what went into constructing the Mishkan. It is strange that the law against using fire on Shabbat is also mentioned.
There is a great deal of discussion as to why this particular law is separated from the rest of the thirty nine מלאכות of Shabbat.
One interesting answer says that the use of fire is called a בנין אב, or a precedent, for all the other מלאכות. Just as fire is forbidden on Shabbat, so, too, are the other מלאכות forbidden.
We find another example of בנין אב with the law of לא תחסום שור בדישו, that we are not allowed to muzzle an ox when threshing. Just like the violation of this negative commandment gives one lashes, lashes would also be given for violating other negative commandments. This is the concept of בנין אב.