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We present moderate-resolution ($R\sim4000$) $K$ band spectra of theplanetary-mass companion VHS 1256 b. The data were taken with the OSIRISintegral field spectrograph at the W.M. Keck Observatory. The spectra revealresolved molecular lines from H$_{2}$O and CO. The spectra are compared tocustom $PHOENIX$ atmosphere model grids appropriate for young, substellarobjects. We fit the data using a Markov Chain Monte Carlo forward modelingmethod. Using a combination of our moderate-resolution spectrum andlow-resolution, broadband data from the literature, we derive an effectivetemperature of 1240 K, with a range of 1200--1300 K, a surface gravity of$\log{g}=$ 3.25, with a range of 3.25--3.75 and a cloud parameter of $\logP_{cloud}=$ 6, with a range of 6.0--6.6. These values are consistent withprevious studies, regardless of the new, larger system distance from GAIA EDR3(22.2$^{+1.1}_{-1.2}$ pc). We derive a C/O ratio of 0.590$_{-0.354}^{+0.280}$for VHS 1256b. Both our OSIRIS data and spectra from the literature are bestmodeled when using a larger 3 $\mu$m grain size for the clouds than used forhotter objects, consistent with other sources in the L/T transition region. VHS1256 b offers an opportunity to look for systematics in the modeling processthat may lead to the incorrect derivation of properties like C/O ratio in thehigh contrast regime.

Moderate-Resolution $K$-Band Spectroscopy of the Substellar Companion VHS 1256 b