I need a simple Excel macro or function that works like the VLOOKUP
function but sums eventual multiple matches.

Example:

Sheet1 - before execution:
DOG   | WHITE
HOUSE | YELLOW
CAT   | GRAY

Sheet2 - before execution:
HOUSE | 300
DOG   | 780
DOG   | 200
BALL  | 560

Sheet1 - AFTER execution:
DOG   | WHITE  | 980
HOUSE | YELLOW | 300
CAT   | GRAY   |

HERE IS THE COMPLETE SOLUTION:

What you really want is called a SUMIF function.
In your Sheet1 cell C1, enter the following formula:
=SUMIF(Sheet2!$A$1:$A$4,A1,Sheet2!$B$1:$B$4)
and then just copy the formula down.

Excellent! it works!
Thank you very much!

Luca