does vacuum rebuild index?

hi,
we have a 34 million row table. after many inserts, updates, deletes
the performance degraded so we vacuumed the table. the output indicated
that the index was also vacuumed but the size didn't change. it was still
80m so we dropped and recreated it reducing it to 20m. no big deal but
is it common for a vacuum to "miss" the index, so to speak? i saw that
someone else asked a similar question on 6/4 in the general archives but
i found no reply to it. is this a common occurrence or am i doing something
wrong or maybe should be doing something different? this is a 7 column
index on an 11 column table.

xxxxxxx1 | varchar(15) |
xxxxxxx2 | bigint |
xxxxxxx3 | varchar(15) |
xxxxxxx4 | varchar(15) |
xxxxxxx5 | bigint |
xxxxxxx6 | float8 |
xxxxxxx7 | float8 |
xxxxxxx8 | float8 |
xxxxxxx9 | date |
xxxxxxx10 | integer |
xxxxxxx11 | bigint |

the index is on columns 1-5,10,11

thanks,
mikeo