I've got an answer from Cisco.
The command "ip flow ..." is only available on Interfaces, which are not configured as switchport.
I've started testing it with a configuration like:
flow timeout 60
flow exporter FlowExp_NetQosServer
description Net Flow Export to NetQosServer
destination 1.2.3.4 use-vrf xxxx
transport udp 9995
source loopback3
version 9
template data timeout 300
option exporter-stats timeout 60
sampler FlowSampler1
description NetFlow TestSampler 1
mode 1 out-of 1
flow monitor FlowMoni_NetQosServer
record netflow-original
exporter FlowExp_NetQosServer
...
interface Vlan1234
...
ip flow monitor FlowMoni_NetQosServer input sampler FlowSampler1
...
It seems this works.
Here is the link to the relevant Cisco Page:
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus6000/sw/system_management/7x/b_6k_System_Mgmt_Config_7x/b_6k_System_Mgmt_Config_7x_chapter_010011.html
Regards, Frank