Putting an SRX into packet-mode, is it possible to configure IPSec VPN tunnel?
No. With Junos Enhanced-Services, or JUNOS 9.4 or higher, IPSec is only supported with flow mode (which is default mode). If you convert the device to packet-mode (e.g. configuring MPLS), you will not be able to configure IPSec VPN. IPSec VPN is only supported if the device is in flow mode.