Vacuum and index

Bonjour,

Loading data (around 112000 record) to a newly created
data-base, I am able to do select such as:

select * from userreg where userid='cynthiaU';

Where 'userid' is defined as:
userid VARCHAR(30) UNIQUE,
in the table.

Everything work fine, as soon I run vacuum
on this data-base some of the records "disappear",
(say 10%) such the previous select is not responding, while

select * from userreg where userid~*'cynthiaU';

is still responding.
Obviously there is an index problem, If drop and
create the index related to the table, everything
comeback fine....

Questions:
1) Why index become mis-aligned after a data-base
vacuum?
2) Is there a way to keep table and index aligned under
vacuum?

A bientot
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