Limited Partners (LPs) are generally not issued units, shares, or any equivalents for their investment into a VC Fund - this is a common misconception. LPs simply own a percentage of the limited partnership's interests. 

By design, the structure of VC Funds makes it very difficult for LPs to pull their money out until there are liquidity events (exits). This is because the capital is directly invested in private companies - money is returned to LPs usually when one of those companies undergoes a liquidity event (acquisition, IPO, etc.).