'Vulgraad' doesn't strike me as standard terminology in this context, but based on experience (in electronics) I would understand it to mean 'component density'. That's plausible because multilayer construction allows the interconneting tracks to be routed 'on top of each other', so they take up proportionally less of the board area -- which allows the components to be placed closer together (higher density).

Sample ref:

Cost-effective layout of microprocessor boards
Multi-layer construction is accepted practice for high performance circuits which use high value components such as 32-bit microprocessors and other VLSI peripherals. It may also be necessary to permit sufficiently high interconnect and component density, or to allow close control of track characteristic impedance. In these cases the much higher cost of multi-layer compared to conventional construction is either a small fraction of the overall product cost or is regarded as a necessary expense to meet the performance requirements.
www.elmac.co.uk/c_e_layt.htm
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