The vortex state of a type-II superconductor produces a distinctive ${\mathrm{\ensuremath{\mu}}}^{+}$SR line shape with features determined by the average internal field ${\mathit{B}}_{\mathit{o}}$, the magnetic penetration depth \ensuremath{\lambda}, the superconducting coherence length \ensuremath{\xi}, and the degree of disorder in the vortex lattice. Only in the high field regime (\ensuremath{\lambda}\ensuremath{\gg}L>\ensuremath{\xi}, where L is the intervortex spacing) do the vortex cores (of radius \ensuremath{\approxeq}\ensuremath{\xi}) occupy a large enough area that they are observable in the line shape as a high field cutoff. Our ${\mathrm{\ensuremath{\mu}}}^{+}$SR measurements of the field distributions in a mosaic of single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ in fields of 1.9, 4.1, 4.7, and 6.5 T (${\mathbf{B}}_{\mathit{o}}$\ensuremath{\equiv}c) are the measurements by \ensuremath{\mu}SR or NMR in a high-${\mathit{T}}_{\mathit{c}}$ superconductor which show all the features of the line shape. We find \ensuremath{\lambda}=0.15\ifmmode\pm\else\textpm\fi{}0.01 \ensuremath{\mu}m at 10 K and the Ginzburg-Landau parameter \ensuremath{\kappa}\ensuremath{\equiv}\ensuremath{\lambda}/\ensuremath{\xi}=69.6\ifmmode\pm\else\textpm\fi{}1.4 constant between 30 and 75 K; this is the only measurement to date of \ensuremath{\kappa} in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ below the irreversibility temperature. Due to disorder in the vortex lattice, either from pinning or from vortex fluctuations that are quasistatic on the time scale of ${\mathrm{\ensuremath{\mu}}}^{+}$SR, the observed line shape is ``smeared'' relative to that predicted for a perfect lattice. From the degree of smearing, we estimate an upper limit of 5.5% for the rms deviation of individual vortices from their ideal positions.