You might want to go back to the well on your 401(a). I have a 401(a) and 457(b) through work. There is some matching for the 401(a). I'm always ready to learn new things, but I believe the 401(a) is a "defined contribution" plan, not a "defined benefit" plan. Unless your employer has a strange and specific vesting requirement, their contributions to your 401(a) should be yours as soon as made. No give-backsies. The only limits on your ability to withdraw from the 401(a) for your own use come from the standard age/tax penalty limits on 401(a) withdrawals (same as 401(k)). So I'm not sure about "never actually getting" the employer contributions if you leave.
A 457(b) is a fantastic plan. You are allowed access to it upon separation from employment without penalty, and it can be an amazing tool in your FIRE toolbox. Between my contribution to my 401(a), and maxing the 457(b), we are reducing household taxable income by about $30,000 annually, which on our income makes us eligible for a Roth. With the exception of contributing to the 403(b) up the employer match, and knowing nothing else about your situation, I think the 457(b) is an excellent idea, particularly if you want to retire well before 59.5. The only thing I would caution is that I believe that the 401(a) and 403(b) have matching caps (e.g., you can max a 401 and a 457, but not also a 403), so you're going to want to investigate further. Assuming your 401(a) maxes at the $19,000 cap in 2019, and the 403(b) with match is a bit over $3,000, I believe you're going to have to limit your 457(b) contribution. Good luck!