${\vec{v}}_2=2{\vec{v}}_1$

${\vec{v}}_{21}={\vec{v}}_2-{\vec{v}}_1$

${\vec{v}}_{21}=2{\vec{v}}_1-{\vec{v}}_1$

${\vec{v}}_{21}={\vec{v}}_1$

$v_{21}=\left|{\vec{v}}_{21}\right|$

$v_{21}=v_1$

${\vec{v}}_{12}={\vec{v}}_1-{\vec{v}}_2$

${\vec{v}}_{12}={\vec{v}}_1-2{\vec{v}}_1$

${\vec{v}}_{12}=-{\vec{v}}_1$

$v_{12}=\left|{\vec{v}}_{12}\right|$

$v_{12}=v_1$