table에서 수직 정렬방법을 찾으시는 듯하여 두가지 예제를 보여드립니다.

\documentclass[a4paper,amsmath]{oblivoir}
\usepackage[a4paper, top=2cm, bottom=2cm, left = 2cm, right = 2cm]{geometry}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{arrows, angles, quotes}
%\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{physics}
\usepackage{array} % added 
\usepackage{tabu}  % added 
\renewcommand{\v}[1]{\mathbf{#1}}
\newcommand{\proj}[2]{\mathrm{proj}_\v{#1}(\v{#2})}

\begin{document} 
    \begin{center} 
        \begin{tabular}{c c} 
            \hline 
            \begin{tikzpicture} 
            \coordinate (o) at (0,0); 
            \coordinate (u) at (1,3); 
            \coordinate (v) at (4,0); 
            \draw[thick, ->] (o) -- (u) node[midway, above left] {$\v{u}$}; 
            \draw[thick, ->] (o) -- (v) node[midway, below] {$\v{v}$}; 
            \draw[ForestGreen, very thick, ->] (o) -- (1,0) node[midway, below] {$\proj{v}{u}$}; 
            \pic [draw, ->, "$\theta$", angle eccentricity=1.5] {angle = v--o--u}; 
            \draw[dashed] (1,0) -- (1,3); 
            \end{tikzpicture} & 
            \(  
            \begin{aligned} 
            \proj{v}{u} &= \norm{\v{u}}\cos\theta \cdot \frac{\v{v}}{\norm{\v{v}}}\\ 
            &= \norm{\v{u}} \cdot \frac{\v{u} \circ \v{v}}{\norm{\v{u}}\norm{\v{v}}} \cdot \frac{\v{v}}{\norm{\v{v}}} \\ 
            &= \proj{v}{u} = \frac{\v{u}\circ\v{v}}{\v{v}\circ \v{v}} \v{v} 
            \end{aligned} 
            \) \\ 
            \hline 
        \end{tabular} 
    \end{center} 
    
% https://ko.overleaf. com/learn/latex/Tables , Tables with fixed length, 
    \begin{center} 
        \begin{tabular}{m{0.25\textwidth} m{0.25\textwidth} } 
            \begin{tikzpicture} 
            \coordinate (o) at (0,0); 
            \coordinate (u) at (1,3); 
            \coordinate (v) at (4,0); 
            \draw[thick, ->] (o) -- (u) node[midway, above left] {$\v{u}$}; 
            \draw[thick, ->] (o) -- (v) node[midway, below] {$\v{v}$}; 
            \draw[ForestGreen, very thick, ->] (o) -- (1,0) node[midway, below] {$\proj{v}{u}$}; 
            \pic [draw, ->, "$\theta$", angle eccentricity=1.5] {angle = v--o--u}; 
            \draw[dashed] (1,0) -- (1,3); 
            \end{tikzpicture} & 
            \(  
            \begin{aligned} 
            \proj{v}{u} &= \norm{\v{u}}\cos\theta \cdot \frac{\v{v}}{\norm{\v{v}}}\\ 
            &= \norm{\v{u}} \cdot \frac{\v{u} \circ \v{v}}{\norm{\v{u}}\norm{\v{v}}} \cdot \frac{\v{v}}{\norm{\v{v}}} \\ 
            &= \proj{v}{u} = \frac{\v{u}\circ\v{v}}{\v{v}\circ \v{v}} \v{v} 
            \end{aligned} 
            \) \\ 
        \end{tabular} 
    \end{center}     
    
% http://conf.ktug.org/2011/Program_files/KTS2011-progress.pdf
    \begin{center} 
        \begin{tabu} to 0.6\textwidth { X[c,m] X[2,c,m] }        
            \begin{tikzpicture} 
            \coordinate (o) at (0,0); 
            \coordinate (u) at (1,3); 
            \coordinate (v) at (4,0); 
            \draw[thick, ->] (o) -- (u) node[midway, above left] {$\v{u}$}; 
            \draw[thick, ->] (o) -- (v) node[midway, below] {$\v{v}$}; 
            \draw[ForestGreen, very thick, ->] (o) -- (1,0) node[midway, below] {$\proj{v}{u}$}; 
            \pic [draw, ->, "$\theta$", angle eccentricity=1.5] {angle = v--o--u}; 
            \draw[dashed] (1,0) -- (1,3); 
            \end{tikzpicture} 
            & 
            \(  
            \begin{aligned} 
            \proj{v}{u} &= \norm{\v{u}}\cos\theta \cdot \frac{\v{v}}{\norm{\v{v}}}\\ 
            &= \norm{\v{u}} \cdot \frac{\v{u} \circ \v{v}}{\norm{\v{u}}\norm{\v{v}}} \cdot \frac{\v{v}}{\norm{\v{v}}} \\ 
            &= \proj{v}{u} = \frac{\v{u}\circ\v{v}}{\v{v}\circ \v{v}} \v{v} 
            \end{aligned} 
            \) \\ 
        \end{tabu} 
    \end{center}      
    
\end{document}