= Solution
Take the retained disc mass to be the <cosmic baryon fraction> of the total <dark matter halo> mass: $M_d=f_bM_h$, where $f_b=0.05/0.25=0.2$. Adopt $v_{\rm vir}^2=GM_h/r_{\rm vir}$ and describe the disc gravity by $v_d^2=C_dGM_d/R_d$. The structure coefficient $C_d$ must be stated: the scale-free <Mestel disc> gives $C_d=1$ when its flat rotation speed and enclosed mass are used, while a finite disc requires boundary and thickness information. The equality of edge <specific angular momentum> gives $R_dv_d=q r_{\rm vir}v_{\rm vir}$, with $q=0.1$. Combining these relations gives the <angular-momentum-conserving self-gravitating disc radius>:
$$
\frac{v_d}{v_{\rm vir}}=\frac{C_df_b}{q}=2C_d,\qquad \frac{r_{\rm vir}}{R_d}=\frac{C_df_b}{q^2}=20C_d.
$$
With the commonly intended $C_d\simeq1$ approximation,
$$
\boxed{v_{\rm vir}\simeq105\,\mathrm{km\,s^{-1}},\qquad r_{\rm vir}\simeq97.0\,\mathrm{kpc}.}
$$
The corresponding masses are $M_h\simeq2.49\times10^{11}M_\odot$ and $M_d\simeq4.97\times10^{10}M_\odot$, where $M_\odot$ is the <solar mass>.
For a definite formation estimate, assume the <dark matter halo> acquired this mass and radius at <virial equilibrium>, retained them afterward, and had mean density $\Delta_v\bar\rho_m(z_f)$ with $\Delta_v=18\pi^2$. This uses the matter-dominated <spherical-collapse model> as an approximation to the specified matter-plus-vacuum cosmology. Since $\bar\rho_m(z_f)=3H_0^2\Omega_{m0}(1+z_f)^3/(8\pi G)$,
$$
\left(\frac{v_{\rm vir}}{r_{\rm vir}}\right)^2=\frac{\Delta_v}{2}H_0^2\Omega_{m0}(1+z_f)^3,
\qquad \boxed{z_f\simeq1.21.}
$$
The approximate distance therefore gives formation modestly before observation. Using the numerical rather than approximate distance integral gives $z_f\simeq1.22$, within the accuracy of these structural assumptions. The calculation also assumes inclination-corrected spectroscopy, negligible disc pressure support, conserved edge <specific angular momentum>, no substantial later accretion or mergers, and no significant baryon loss.
\b[This formation redshift is model-dependent, not uniquely fixed by the stated disc data.] Keeping $C_d$ explicit gives $1+z_f\simeq2.209C_d^{-4/3}(\Delta_v/18\pi^2)^{-1/3}$. In particular, replacing the mean-density convention by $200$ times the <critical density> gives $H(z_f)=v_{\rm vir}/(10r_{\rm vir})$ and $z_f\simeq0.87$, later than the observation. That inconsistent chronological result cannot be silently used as the formation epoch.
There is a further idealization in applying $C_d=1$ exactly at the edge: an abruptly truncated <razor-thin disc> with nonzero edge <surface density of a disk> has a logarithmically divergent in-plane force at that edge. Locally the available mass occupies a half-plane, and its radial contribution is proportional to $G\Sigma_d\int_{\epsilon}^{\ell}ds/s$, with no opposite exterior half-plane to cancel it. Finite thickness or a smooth taper removes that <sharp-edge force singularity of a thin disk>. Thus $C_d=1$ is an explicitly qualified <Mestel disc> or monopole-scale approximation, not an exact finite-disc calculation; <enclosed mass does not determine a disc rotation curve>.
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