what you can try is setting the tcp-mss on the vpn to somthing like 1350.
marcel@srx09.lab # set security flow tcp-mss ipsec-vpn mss 1350
You can also try sending over packets with a max size of 1500 over the vpn and lower the value until you reach the size that will "pass" the vpn. You can use the max packet size then to set that as the max for the ipsec-vpn mss
marcel@srx09.lab> ping <ip on otherside of the vpn> size 1500
marcel@srx09.lab> ping <ip on otherside of the vpn> size 1420