Running vacuum while executing the application

Hi!

My application in C++ use a Postgres Database. I'm executing vacuum once a
day in my database with the application running at the same time (and
interacting with Postgres).
When I execute vacuum, I obtain this error in my application (while trying to
execute a query in Postgres):

[unixODBC]Error while executing the query (non-fatal);
ERROR: RelationClearRelation: relation 1233323 modified while in use

Is there any problem to execute vacuum and trying to access to the database
at the same time?

Thank you.

Raquel