The alternative is to to take an eight hour slowboat up the Nam Ou river to Nong Kiaow/ Muang Ngoi and then overnight at Nong Kiaow. From there it will be overland to Houay Xai via Udomxai. I would probably need to allow three days for this alternative route hence one day less in Vang Viang.
Places to stay in Nong Kiaow that look interesting are Sunset Guesthouse and Sengdao Bungalows and Restaurant