can you use retirement funds to invest into VC funds

Possibly - however it really depends on what kind of retirement funds/accounts are being contemplated, and where and how those funds/accounts are held. Country-specific regulations may also apply, as a general rule of thumb.

For example, Delaware-domiciled funds working with Decile Partners may be able to accept certain retirement funds via a certain type of trust company vendor (such as Strata Trust Company, a partner) - however, doing so takes additional setup on the part of the Fund, as well as the limited partner in question.

TL:DR - possible depending on jurisdiction and trust company (as applicable), but more complicated and may take longer process-wise.