** the metal tube that functions as the dryers exhaust pipe is not tight to the plastic vent cover on the outside of the house. How can I fix this? **
Many vent covers come with an attached sleeve (see the following link) which should allow connecting to a recessed vent line. When connecting it they are usually just press fit but could be taped.
Screws should NOT be used as their protrusions into the pipe will collect lint which may eventually accumulate to the point where it will lead to a blockage. They will also make cleaning (which should be done at least once a year) more difficult.
** As a result, a small amount of lint has collected between the edge of the metal pipe and the AL siding on the house. Should I be concerned with moisture as a result of this flaw? **
It may eventually cause rot problems with the under laying wood in the area not to mention air leakage and the likely reduction in insulating value.