Hi,
I've finally managed to boot VxWorks from linuxbios using old (non-elfboot) method. This required placing linuxbios itself to other memory location (0xC0000 instead of 0x4000) and reducing it's heap size to 64K, so that data, bss, heap and stack could be placed between 0xC0000 and 0xF0000. To make these areas mapped as DRAM and not PCI, I had to initalize PAM registers differently.
After I've managed to relocate linuxbios code, booting vxworks was easy. The VxWorks bootloader is uncompressed to 0x8000 and jump is performed to 0x8010 (protected mode entry point).
If anyone is interested in more details, I'll be glad to provide them.
I'd like to specially thank Peter Fox and Cristopher Stutts for their help.
Felix.