Vietnam conflict casualties are recorded in an online database (https://aad.archives.gov/aad/show-more-fields.jsp?dt=2513&dl=1610&tf=F&cat=WR28&bc=,sl&sc=27435,27438,27439,27445,27447,27448,27450,27468,27423) (hope the link works) provided by the National Archives. Unfortunately the interface is incredibly awkward and there is no easy way to see casualties by month.

The Archives also provides a download of the full database of casualties, which would be easier to crunch to get your stats. It's at:

https://catalog.archives.gov/id/2240992?q=*:*

Good luck!

To follow up my previous post, I crunched the numbers on a local copy of the Casualty database and got the following casualties by month for the entire conflict:

Casualties, Month

2416 196805

2259 196802

1767 196803

1579 196804

1552 196903

1545 196801

1455 196905

1396 196705

1313 196808

1301 196902

1254 196806

1244 196906

1183 196809

1132 196711

1091 196703

1056 196707

1021 196904

991 196807

985 196901

959 196908

948 196706

936 196712

929 196709

917 197005

916 196811

904 196710

885 196704

855 196812

833 196907

821 196810

766 196702

730 197004

683 196708

663 196909

658 196911

641 196611

637 196701

624 197003

600 196910

595 196606

591 196605

568 197002

563 196603

561 196607

560 197006

545 196511

533 196612

532 196609

529 197001

516 196610

512 196602

509 196912

500 196608

495 197007

452 197008

429 197103

405 196604

401 196601

385 197102

363 197009

347 197011

326 196512

317 197010

292 197104

271 197012

259 197101

236 196510

202 197105

181 197106

179 196508

150 196509

146 196506

132 197110

127 197109

118 197107

116 197108

113 197205

109 197111

106 197204

103 196507

 89 196505

 88 197206

 75 197212

 68 197203

 65 197210

 64 197112

 60 196504

 54 196502

 53 197207

 47 197201

 46 197202

 42 197505

 41 197208

 36 197209

 36 196412

 29 197301

 29 196410

 24 196310

 23 196312

 22 196503

 21 197211

 19 196411

 18 196501

 18 196406

 17 196408

 17 196407

 17 196403

 16 197504

 15 196405

 14 197302

 14 196401

 14 196301

 12 196404

 12 196202

 11 196409

 11 196402

 11 196210

 10 196308

  8 197306

  8 196302

  7 196311

  7 196307

  7 196304

  7 196207

  7 196103

  6 197304

  6 196305

  6 196303

  6 196204

  5 196309

  5 196306

  4 197501

  4 196211

  4 196104

  4 196002

  3 196206

  3 196205

  3 196201

  2 197309

  2 197307

  2 197305

  2 196212

  2 196108

  2 195907

  1 200605

  1 200601

  1 200503

  1 200502

  1 200409

  1 199011

  1 198709

  1 197411

  1 197312

  1 197311

  1 197310

  1 197308

  1 197303

  1 196208

  1 196203

  1 196112

  1 196106

  1 196101

  1 196008

  1 195710

  1 195606