Exactly, u will NOT lose current credit provided you do a top up again just before the 9 months of inactivity period is due. Doing so u reactivate the account and also the credit you had frozen during SIM inactivity period. Since no Polish operator vouchers could be used and there's no offers on Ebay to top up Vipnet I could start a small business of offering Croatian networks' vouchers online
which should work rather well given the number of tourists such as urself who visit Croatia once a year for summer holidays.
There is a classified ads online site
Njuskalo.hr oglasnik but don't think it's of use 2 u because private sellers can't accept credit card payments from abroad and I'm not sure if Paypal or similar services offer to receive payments in Croatia.