vusernumber looks like a variable, not a field name.
The "too few parameters" error occurs when field names are unrecognized.
You need to seperate variables outside of the SQL, and concatenate them using &'s.
If vusernumber is numeric, use this:
SqlStr = "INSERT INTO OT_table ([OT_number], [date_open]) VALUES (" & vusernumber & ", #" & Date & "#);"
If it is Text, use this:
SqlStr = "INSERT INTO OT_table ([OT_number], [date_open]) VALUES ('" & vusernumber & "', #" & Date & "#);"