Custom environment won't use align instead of eqnarray

I have the following minimal example in which I define a new environment named derivation. Initially, I used eqnarray in the definition of my new environment, but I now want to use align. When I do so, I get an error message saying "\begin{align} on input line 19 ended by \end{derivation}. How can I fix this? (Line 19 is \begin{derivation})

% !TEX TS-program = pdflatexmk
% Minimal Template Version 20120522




\vectsub{v}{avg} &= \DbyDt[\vect{r}] \\
\Dvect{r} &= \vectsub{v}{avg} \Changein{t} \\
\vectsub{r}{final} - \vectsub{r}{initial} &= \vectsub{v}{avg} \Changein{t} \\
\vectsub{r}{final} &= \vectsub{r}{initial} + \vectsub{v}{avg} \Changein{t}