initially VTiger is setup to not allow for manual account number entry. the easiest way I have found to get around this is as follows:

mysql -u root -p
update vtiger.vtiger_modentity_num set prefix='', start_id='', cur_id='', active='0' where vtiger_modentity semodule='Accounts';
UPDATE vtiger.vtiger_field SET uitype = '2' WHERE vtiger_field.columnname ='account_no' ;