How do I determine the correct index on MS-SQL-Server based on a select?
You are unlikely to ever see index use because of this in the query:
CONVERT(VARCHAR(6), v1.Tag, 112)

Because it has to calculate the values from v1.Tag, it can't simply use the histogram in the statistics to determine row counts. It must scan the entire table to find all the values that match, and it has to do the conversion each time.
If u1.Monat and v1.Tag are not the same data type, adding a column that is the same data type might be necessary (on either u1 or v1, whichever).
However, the bigger issue is, you're moving all the data. The query has no WHERE clause to filter anything. Without a filter, the most efficient way to move everything is through scans. Index seeks wouldn't make sense. You might see some index usage on the JOINs between tables (once you fix the CONVERT problem), but possibly not since all the rows are being moved.