After unstaking your LP from NFTX you will need to unwind your SLP position on the SushiSwap site.
NFTX is unable to unwide the SLP position as part of the unstake liquidity process, so you may find yourself left with SLP tokens.
The SLP tokens are a claim on the ETH and vTokens you have on SushiSwap.
To unwind the position you can go directly to https://app.sushi.com/legacy/remove/ETH/<vToken>?chainId=1 where <vToken> is the vault address/token.
You can get this by checking the info tab on the vault, or by copying it from the URL on the vault page.